Set JSON as an explicit dependency

Fixes the error when trying to launch coffeelint with ruby 2.0.0 (ruby-install, chruby, ubuntu 12.04.2 x86)

```bash
uninitialized constant Coffeelint::JSON
/home/andrei/.gem/ruby/2.0.0/gems/coffeelint-0.1.2/lib/coffeelint.rb:27:in `lint'
/home/andrei/.gem/ruby/2.0.0/gems/coffeelint-0.1.2/lib/coffeelint.rb:36:in `lint_file'
/home/andrei/.gem/ruby/2.0.0/gems/coffeelint-0.1.2/lib/coffeelint.rb:42:in `block in lint_dir'
/home/andrei/.gem/ruby/2.0.0/gems/coffeelint-0.1.2/lib/coffeelint.rb:41:in `glob'
/home/andrei/.gem/ruby/2.0.0/gems/coffeelint-0.1.2/lib/coffeelint.rb:41:in `lint_dir'
/home/andrei/.gem/ruby/2.0.0/gems/coffeelint-0.1.2/lib/coffeelint.rb:75:in `run_test_suite'
/home/andrei/Work/salt/saltjs/Rakefile:21:in `block (2 levels) in <top (required)>'
```