Pin bundler version to 1.13.7 to avoid breaking changes

In GitLab 8.16.3, bundler was quietly upgraded from 1.14.3 from 1.13.7. 1.14.3 has this change that caused gitlab-rake commands to fail: https://github.com/bundler/bundler/commit/02e7f67727b45f59ec0aec4df410e05921d94928

Currently /var/opt/gitlab is owned by root:root, but bundler 1.14.x expects the user git to be able to write to that directory. We should consider changing omnibus to support this.

Closes #1913 (closed)