use bundler_args vs a custom install step

If you use a custom install step Travis will not use travis_retry, it's little shell function for retrying commands if they fail.
Also removed the 'gem update bundler' as bundler is up to date, and this step adds extra work which is not needed.
Merge pull request #1 from joshk/patch-1

use bundler_args vs a custom install step. Thanks to joshk for the fix!