Remove rails dependency

## Not *Rails* dependency

By removing the ```rails``` dependency on your *gemspec* file, the project using ```premailer-rails``` gem won't update it's *rails* major, minor, patch version by just installing the last *premailer-rails* version available (i.e using ```bundle update premailer-rails```for example).
This are good practices used in projects like [devise](https://github.com/plataformatec/devise).