Minor fix in Travis config with bundler call (#2806)

* Travis: Fix bundler_args

`bundler_args` is not functional if using custom install script
in `.travis.yml`. Directly attach the argument to the install script.

* Travis: Run i18n-tasks through bundle exec

Parallel to similar scripts in the same travis config.
More resiliant to changes in configuration.

* Travis: bundler vendor directory

Travis + rvm seems to ignore `.bundle/config` and hence use rvm
global vendor directory by default. Adding `--path` will fix this
and hence make `cache.bundler = true` really functional.

* Travis: disable bundler cache

No significant change in build time. Huge cache. Best disabled.