Update .travis.yml

Removing rbx for now seems like its not being installed by travis-ci 
Using worker: worker-linux-10-1.bb.travis-ci.org:travis-linux-16
git.1
$ git clone --depth=50 --branch=master git://github.com/hammernight/taza.git hammernight/taza
Cloning into 'hammernight/taza'...
remote: Counting objects: 607, done.
remote: Compressing objects: 100% (278/278), done.
remote: Total 607 (delta 321), reused 542 (delta 294)
Receiving objects: 100% (607/607), 88.88 KiB | 0 bytes/s, done.
Resolving deltas: 100% (321/321), done.
Checking connectivity... done.
$ cd hammernight/taza
git.3
$ git checkout -qf 2d06bd4f21cf2ca7b85dfdad266d75146b9a6217
$ rvm use rbx --install --binary --fuzzy
rbx is not installed - installing.
Searching for binary rubies, this might take some time.
Requested binary installation but no rubies are available to download, consider skipping --binary flag.
Gemset '' does not exist, 'rvm rbx do rvm gemset create ' first, or append '--create'.
The command "rvm use rbx --install --binary --fuzzy" failed and exited with 2 during setup.
Your build has been stopped.