Modify for travis

[rvm 2.0.0]
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.5.1) ruby
  Current Bundler version:
    bundler (1.3.5)

[rvm 2.1.0]
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.5.1) ruby
  Current Bundler version:
    bundler (1.5.0)