Add retry option of travis and bundler

What does this MR do?

This PR adds the retry option for bundler and travis. Bundler now tries 3 times to install the bundle. If the command fails Travis tries the command again.

Are there points in the code the reviewer needs to double check?

These options are documented [here](http://docs.travis-ci.com/user/build-timeouts/)

Why was this MR needed?

Currently often times out when installing gems. This makes Travis pretty useless.

What are the relevant issue numbers?

Fixes https://github.com/gitlabhq/gitlabhq/issues/7416