explicitly use bundler 1.5.3 to avoid vagrant dep problems

was seeing the following in travis build:

> Bundler could not find compatible versions for gem "bundler":
>   In Gemfile:
>     vagrant (>= 0) ruby depends on
>       bundler (~> 1.5.2) ruby
>   Current Bundler version:
>     bundler (1.6.1)

see: https://travis-ci.org/phinze/landrush/builds/24335968