Support travis_retry function

Travis provides a `travis_retry` function that eases retrying commands (in particular for temporary networking issues).

> […] you can wrap commands using the function `travis_retry` which checks the return code of a command, retrying it three times if the return code is non-zero.
>
> ```
> install: travis_retry bundle install
> ```

http://docs.travis-ci.com/user/build-timeouts/#Timeouts-installing-dependencies

See https://github.com/travis-ci/travis-build/pull/104 for the source of the function definition.

Support for this function would be nice.