Force the bundler version to override possible newer versions

If multiple versions of bundler are installed the latest version will be used regardless of the version specified in BUNDLER_VERSION. Bundler lets you use an older version by providing it as the first argument. (Surrounded by underscores)

```
bundle _1.5.2_ install
```

instead of

```
bundle install
```

This PR adds BUNDLER_VERSION as the first arg during the install step. I tested it on my heroku app where 1.6.1 and 1.5.2 both exist at the moment, and I needed to force the use of 1.5.2.