Escape the travis-ci trouble when installing ruby 2.1.0

See https://github.com/travis-ci/travis-ci/issues/2220
Change TravisCI Ruby version from 2.1 to 2.1.1

Additinally, add 2.1.0 in allow-failure.
Add TravisCI Ruby version 2.1.0 to .travis.yml

This patch force compiling on MRI 2.1.0
Follow the MRI version rules
Merge pull request #76 from kaznum/travis-ruby-2.1.0-trouble

Escape the travis-ci trouble when installing ruby 2.1.0