Revert "Travis ci: Use the ruby directive in the Gemfile"

This seems not to be working. I get:

ruby-2.1.5 is not installed.

Could not determine which Ruby to use; . should contain .rvmrc or
.versions.conf or .ruby-version or .rbfu-version or .rbenv-version, or
an appropriate line in Gemfile. ( see: 'rvm usage' )

This reverts commit 3219040e0626649d05b2371e9f71fedd43ecb53f.