Specify ruby-1.8.7-p374 as possible workaround for Travis error.

Currently it's defaulting to ree-1.8.7-2012.02and failing to find rake
with 'Could not find rake-0.9.2.2 in any of the sources' even though the
gem has been installed.

https://s3.amazonaws.com/archive.travis-ci.org/jobs/24249705/log.txt