Two PRs have been merged that turned out to be slightly incompatible. This PR fixes the failing tests.
Root causes:
* The pipeline parameter default values were not properly serialized when constructing the metadata object.
* The `ParameterMeta` class did not validate the default value type, so the lack of serialization has not been caught. The `ParameterMeta` was replaced by `InputSpec` which has strict type validation.
* Previously we did not have samples with complex pipeline parameter default values (e.g. lists) that could trigger the failures. Then two samples were added that had complex default values.
* Travis does not re-run tests before merging
* Prow does not re-run Travis tests before merging
6afb91b902