Fix substring replacement

https://github.com/steida/gulp-closure-deps/blob/master/index.js#L53

Replace takes a substring or a function as a second argument. If you pass a string then certain 'replacement rules' apply such as `$$, $&` etc.

Obviously we don't want this.

Example - i got module named `client$$` which became `client$` in deps.js since $$ is replaced by a single $ by these replacement rules :)