docs(README): add Successfully installed sass-3.4.14

[nbrustein]

Parsing documentation for sass-3.4.14
Installing ri documentation for sass-3.4.14
Done installing documentation for sass after 5 seconds
1 gem installed step to devserver setup instructions

I hit the following error when running gulp dev. installing the sass gem fixed it, so just adding that to the readme:

[14:51:41] [gulp-ruby-sass] Error: spawn ENOENT

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:1000:11)
    at Process.ChildProcess._handle.onexit (child_process.js:791:34)