before_install:
  - gem uninstall bundler
  - gem install bundler -v 1.6.2

before_install:
  - gem uninstall bundler -a
  - gem install bundler -v 1.6.2

Remove all `bundler` versions before CI build
Remove all `bundler` versions before CI build

Looks like Travis still have more bundler versions and try to use newer
one than expected.
