Don't use BUNDLE_JOBS for now

Install fails in Rubinius when bundler runs with more than one job. I
tried to figure out how to make the matrix work to allow BUNDLE_JOBS=4
for all the other rubies, but I couldn't get it working.

Locally Rubinius installs with BUNDLE_JOBS=4 / --jobs=4 just not on
Travis. Your guess is as good as mine.

Ping: @rkh and all the fantastic people at Travis.