Fixed travis by updating Gemfile to pin Rake to 10.1.1

The latest Rake update requires Ruby >= 1.9. This update
fixes the failing 1.8.7 tests by pinning Rake to the last
supported version on ruby 1.8.7.

This patch is borrowed from @blkperl
Merge pull request #398 from igalic/rake

Fixed travis by updating Gemfile to pin Rake to 10.1.1