Supresses a warning when using bundler

Not sure if older versions of bundler did this as well in, but at least in recent versions there is always at least one gem at a local path used: The gem where bundle is just executed itself.

`Using warbler (1.4.1.dev) from source at .`

This always throws the warning, that this feature is not supported, but in that case I don't think this is necessary.
I can already see it happening when I run the `warbler` specs. All `Warbler::Config` specs output the following:

```
warning: Bundler `path' components are not currently supported.
The `warbler-1.4.1.dev' component was not bundled.
Your application may fail to boot!
```

This little addition supresses the warning when the gem at `'.'` is encountered.