Remove broccoli-replace in favor of broccoli-string-replace.

`broccoli-replace` brings a significant number of dependencies
(including `js2coffee` and `coffee-script`). This represents 15% of the
total `node_modules/` size of `ember-cli`.

`broccoli-string-replace` depends on `broccoli-filter` only, and
lets `String.prototype.replace` do all of the hard work.

```
% du -csh node_modules/broccoli-replace
 20M    node_modules/broccoli-replace
 20M    total
```

```
% du -csh node_modules/broccoli-string-replace
388K    node_modules/broccoli-string-replace
388K    total
```
Merge pull request #1345 from rjackson/remove-broccoli-replace

Remove broccoli-replace in favor of broccoli-string-replace.