Fix: executable paths and bundle install

Hello, 2 small fixes.

1. Wrong executable path:
the repo https://github.com/berkshelf/berkshelf-api.git doesn't contain vendor/ directory. berks-api is located in /bin/berks-api

2. Bundle update does not run on initial install.
Gemfile and Gemfile.lock have same create time, preventing berks-api-bundle-install block from executing.