Unstash stashed changes before exiting

Changes introduced to the app are cleaned up during deployment,
making them unavailable to after_deploy block.
Since such changes include the bundled gems, after_deploy block
could not use "bundle exec" to interact with the application in
an expected manner.
Merge pull request #149 from travis-ci/ha-bug-unstash-for-after-deploy

Unstash stashed changes before exiting