add Ruby 2.0 to .travis.yml

Rails 3 on Ruby 2.1.1 is broken.
http://www.redmine.org/issues/16194
remove branches from .travis.yml

It is too hard to run travis on forked repositories.
It disturbs pull request process.

travis does not run Pull Request.
Gemfile: remove prototype
Gemfile: awesome_nested_set: use github 2-1-stable branch
remove NestedSet patch from config/initializers/10-patches.rb