The 2.1.3 release of go-jose.v2 contains a bug fix for a nil panic encountering null values in JWS headers that affects Boulder. This commit updates Boulder to use the 2.1.3 release. Unit tests were confirmed to pass: ``` $ go test ./... ok gopkg.in/square/go-jose.v2 13.648s ok gopkg.in/square/go-jose.v2/cipher 0.003s ? gopkg.in/square/go-jose.v2/jose-util [no test files] ok gopkg.in/square/go-jose.v2/json 1.199s ok gopkg.in/square/go-jose.v2/jwt 0.064s ```  | 
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