Update credentials plugin

The credentials plugin 1.9.2 through 1.9.3 generates a JavaScript error on form pages that have the <credentials:select/> jelly tag.

1.9.4 [fixes the issue](https://github.com/jenkinsci/credentials-plugin/commit/07978d5c546329ff3301e28a11821897ee461d4a), which only affects Jenkins versions form 1.52x ish onwards