Check if Rails::Railtie class is defined

Old versions of Rails don't support Railtie. Which breaks simply requiring `coffeelint`.

Usually gems with railties suggest users do an explicit `railtie` require in their Gemfile like.

``` ruby
# Gemfile
gem "coffeelint", require: "coffeelint/railtie"
```

But that would be a backwards incompatible change.