Remove rspec duplicate dependency, remove Ruby 1.8.7, add 2.0.0 and 2.1.1 to Travis build matrix

Duplicate rsec dependency raise this notice while gem updating:
```
role_model at /Users/oleg/.rvm/gems/ruby-2.1.1/bundler/gems/role_model-d273b9fc2fa9 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on rspec (~> 2, development), (~> 2) use:
    add_runtime_dependency 'rspec', '~> 2', '~> 2'
```

Nokogiri gem depend Ruby higher than 1.8.7 and raise error on Travis CI.