Add .cache to .gitignore

This should fix the CI failures — the cause here of which is that
Bikeshed generates a .cache directory containing some files that
Bikeshed uses internally. So we need to tell git to not let those files
be committed. Otherwise, without this change, spec-prod appears to be
committing those files to the publishing branch (gh-pages) but then is
subsequently unable to overwrite them.

Author: sideshowbarker

.gitignore

@@ -1 +1,2 @@
 .DS_Store
+.cache