Revert "use 2.1-head in .travis.yml"

This reverts commit 70b023025d488dfa667e6ff06f258c8457238ff2.

https://travis-ci.org/opf/openproject/jobs/19646051

<pre>
$ rvm use 2.1-head --install --binary --fuzzy
/home/travis/.rvm/scripts/selector_parse: line 12: __list_remote_ruby_for: command not found
Using /home/travis/.rvm/gems/ruby-2.1.0
$ export BUNDLE_GEMFILE=$PWD/Gemfile
$ ruby --version
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-linux]
$ rvm --version
rvm 1.25.14 (version) by Wayne E. Seguin <xxx>, Michal Papis <xxx> [https://rvm.io/]
</pre>