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

`broccoli-replace` brings a significant number of dependencies
(including `js2coffee` and `coffee-script`). This represents 14% of the total
`node_modules/` size of `ember-cli` (20MB of 142MB).

`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
```