Upgrade debugger for Ruby 2.1
Update all non-pinned dependencies
Relax version check on Rack
Relax version dependencies on Haml and Sass

This effectively upgrades us to Haml 4.
Run tests on Travis CI on Ruby 2.1.1
Update rspec to version 2.14.x
Bump year in copyright statement
Tidying CHANGES prior to release of 0.10.0
Removed unused optional argument to #path_to
Automate release testing on multiple Rubies