Add Gemfile.lock for stable gem version (e.g. rspec)
fix broken tests by updated ruby and rspec version
travis: test also with ruby 2.1.2
remove ignoring Gemfile.lock
bundler: call bundle install with --retry 3

Rubygems and bundle have often enough temporary problems, so retry 3 times
to be sure we have a permanent problem.
rails: install also bin directory (needed e.g. for rails executable)
bundler: libv8 needs python2.7