This updates Boulder's vendored dependency for `github.com/google/certificate-transparency-go` to c25855a, the tip of master at the time of writing.
Unit tests are confirmed to pass:
```
$ git log --pretty=format:'%h' -n 1
c25855a
$ go test ./...
ok github.com/google/certificate-transparency-go (cached)
ok github.com/google/certificate-transparency-go/asn1 (cached)
ok github.com/google/certificate-transparency-go/client 22.985s
? github.com/google/certificate-transparency-go/client/configpb [no test files]
? github.com/google/certificate-transparency-go/client/ctclient [no test files]
ok github.com/google/certificate-transparency-go/ctpolicy (cached)
ok github.com/google/certificate-transparency-go/ctutil (cached)
? github.com/google/certificate-transparency-go/ctutil/sctcheck [no test files]
? github.com/google/certificate-transparency-go/ctutil/sctscan [no test files]
ok github.com/google/certificate-transparency-go/dnsclient (cached)
ok github.com/google/certificate-transparency-go/fixchain 0.091s
? github.com/google/certificate-transparency-go/fixchain/chainfix [no test files]
ok github.com/google/certificate-transparency-go/fixchain/ratelimiter (cached)
ok github.com/google/certificate-transparency-go/gossip (cached)
? github.com/google/certificate-transparency-go/gossip/gossip_server [no test files]
ok github.com/google/certificate-transparency-go/gossip/minimal 0.028s
? github.com/google/certificate-transparency-go/gossip/minimal/configpb [no test files]
? github.com/google/certificate-transparency-go/gossip/minimal/goshawk [no test files]
? github.com/google/certificate-transparency-go/gossip/minimal/gosmin [no test files]
ok github.com/google/certificate-transparency-go/gossip/minimal/x509ext (cached)
ok github.com/google/certificate-transparency-go/ingestor/ranges (cached)
ok github.com/google/certificate-transparency-go/jsonclient 0.007s
ok github.com/google/certificate-transparency-go/logid (cached)
ok github.com/google/certificate-transparency-go/loglist (cached)
? github.com/google/certificate-transparency-go/loglist/findlog [no test files]
ok github.com/google/certificate-transparency-go/loglist2 (cached)
? github.com/google/certificate-transparency-go/preload [no test files]
? github.com/google/certificate-transparency-go/preload/dumpscts [no test files]
? github.com/google/certificate-transparency-go/preload/preloader [no test files]
ok github.com/google/certificate-transparency-go/scanner 0.009s
? github.com/google/certificate-transparency-go/scanner/scanlog [no test files]
ok github.com/google/certificate-transparency-go/tls (cached)
ok github.com/google/certificate-transparency-go/trillian/ctfe (cached)
? github.com/google/certificate-transparency-go/trillian/ctfe/configpb [no test files]
? github.com/google/certificate-transparency-go/trillian/ctfe/ct_server [no test files]
? github.com/google/certificate-transparency-go/trillian/ctfe/testonly [no test files]
ok github.com/google/certificate-transparency-go/trillian/integration 0.023s
? github.com/google/certificate-transparency-go/trillian/integration/ct_hammer [no test files]
? github.com/google/certificate-transparency-go/trillian/migrillian [no test files]
? github.com/google/certificate-transparency-go/trillian/migrillian/configpb [no test files]
ok github.com/google/certificate-transparency-go/trillian/migrillian/core (cached)
? github.com/google/certificate-transparency-go/trillian/mockclient [no test files]
ok github.com/google/certificate-transparency-go/trillian/util (cached)
ok github.com/google/certificate-transparency-go/x509 (cached)
? github.com/google/certificate-transparency-go/x509/pkix [no test files]
? github.com/google/certificate-transparency-go/x509util [no test files]
? github.com/google/certificate-transparency-go/x509util/certcheck [no test files]
? github.com/google/certificate-transparency-go/x509util/crlcheck [no test files]
```
Daniel McCarney