remove support for ruby1.8, and add support for ruby2.0

This gem is dependent on 'shoulda-matchers' gem, and shoulda-matchers is now incompatible with ruby1.8.
So, currently travis tests for whenever is failing, because bundler throws Gem::InstallError while installing shoulda-matchers in ruby1.8.7 environment.

This commit remove ruby1.8 support from .travis.yml and README.md.

Also, I added ruby2.0 support in README.md. (.travis.yml already includes ruby2.0 support)