Help out succeed Travis timeout errors while bundle install

This allows Travis to retry up to 3 times the `bundle install` command and is very useful to avoid Errored builds due to connectivity issues to rubygems. e.g. [timeout example here](https://travis-ci.org/elgalu/dolarblue/jobs/15508293#L64) that worked thanks to [this](https://github.com/elgalu/dolarblue/blob/master/.travis.yml#L10)