Add License Finder.

[License Finder](https://github.com/pivotal/LicenseFinder) checks the licenses of the gems and npm packages we are using to
make sure they do not conflict with the project. The config/license_finder.yml
contains the whitelisted licenses and the ignored Gemfile groups.

To use license_finder, simply run `license_finder`.

License Finder also checks NPM packages specified in `package.json`, but [there are some issues right now with it](https://github.com/pivotal/LicenseFinder/issues/65). I forked the gem and temporarily removed NPM support. Once it is working and good to go in the official gem repo, I will switch the gem source back to Pivotal's repo.

If there are other licenses we should whitelist, feel free to share them here.

Running the `license_finder` right now currently yields:

```
Dependencies that need approval:
jwt, 0.1.8, other
kgio, 2.8.1, LGPL
pg, 0.17.0, BSD
raindrops, 0.12.0, LGPL
unicorn, 4.6.3, GPLv2
unicorn-rails, 1.1.0, other
```

We should keep track of dependencies that need approval and see if we can whitelist that license. If not, then we will have to figure out what to do from there.