Fix invalid regular expressions in removePaths function.

Just started using gulp-ruby-sass and got an invalid regular expression from file paths in the var folder.
I believe the expression needs to be escaped.
```
/node_modules/gulp-ruby-sass/index.js:46
		msg = msg.replace(new RegExp(path + '/?', 'g'), '');
		                  ^
SyntaxError: Invalid regular expression: //var/folders/NR/NRG9Uc5BH-elQL4jYAdgLk+++TI/-Tmp-/gulp-ruby-sass/?/: Nothing to repeat
    at new RegExp (<anonymous>)
    at /gulp-ruby-sass/index.js:46:21
    at Array.forEach (native)
    at removePaths (/gulp_angular/node_modules/gulp-ruby-sass/index.js:45:8)
    at Socket.<anonymous> (/node_modules/gulp-ruby-sass/index.js:118:14)
    at Socket.emit (events.js:95:17)
    at Socket.<anonymous> (_stream_readable.js:764:14)
    at Socket.emit (events.js:92:17)
    at emitReadable_ (_stream_readable.js:426:10)
    at emitReadable (_stream_readable.js:422:5)
```
use: https://github.com/sindresorhus/escape-string-regexp