Install gems in a shared dir

Set GEM_HOME to a shared directory

Save up time when running `bundle install` by using a shared directory
which is common to all jobs running on a slave.  We did so with change
https://gerrit.wikimedia.org/r/#/c/112682/ for the other browser tests.

Paired with Željko Filipin.

Bug: 58040
Change-Id: Ib7893a678379b591c066d22aa48f2539d8d4b57c
Reference: https://github.com/bundler/bundler/issues/2856
Make bundle install verbose!

Increasing verbosity should help diagnosis when looking at Jenkins
console output.

Paired with Željko Filipin.

Bug: 58040
Change-Id: Ie2f17996e601b87ddfac1112d24daa8bd763a4b8
Use gem 1.9.3 and the installed bundler version

Ubuntu Precise uses gem 1.8.x by default which does not play nice when
installing bundle and we need a 1.9.3 compatible version.

Get bundler installed under $WORKSPACE/vendor and use that version by
pointing GEM_PATH there.

We did a similar hack previously on the Wikimedia installs.

Paired with Željko Filipin.

Bug: 58040
Change-Id: I9b10b2bcd04de1fd7fc3df74094aa3f3399bade9