Add MRI 2.1.? platform to Gemfile

The corresponding Bundler platform for MRI Ruby 2.1.? is `:mri_21`.
Without this, bundler will not install gems like `iconv`, `simplecov`
and `debugger` for Ruby 2.1.?.

Tested:
- Running bundle install with Ruby 2.1.1-p76
- rake test:units
- rake test:functionals
- rails s