bump to v91
changelog
fix stash checking for buildpack:increment
add messaging around buildpack increment success
Merge pull request #204 from heroku/fix_buildpack_increment

Fix buildpack increment
raise an error if problems detecting the bundler vendor directory
figure out slug vendor base without munging in errors
add tests for user env compile
Merge pull request #203 from heroku/stderr

Fixes #202
Merge remote branch 'upstream/master'