Revert "replaced Ruby 1.9.2 with 2.0 in Travis CI configuration"

this was a bit premature:
https://travis-ci.org/innoq/iqvoc/jobs/5158580

I forgot we need to wait for Rails 3.2.13

This reverts commit 93b9923e4d86f34e943a4352777f4317645f305c.