barclamp_install.rb: Exit with an error message when rake tasks fail
Make sure gemfile_plugin is actually synced to disk

With the old code it could happen that the contents of the file weren't
present at the time when the railties:install:migrations rake task was
executed. (Which was leading to installation failures later on)
Merge pull request #1754 from rhafer/master

Make sure gemfile_plugin is actually synced to disk before rake task are executed