Fix browser name

Karma needs a browser definition that is case-sensitive.

This PR fixes:
```console
npm test

> angular-slider@0.3.2 test /Users/joscha/Development/venturocket-angular-slider
> karma start --single-run --no-auto-watch

WARN [karma]: Port 9876 in use
INFO [karma]: Karma v0.10.10 server started at http://localhost:9877/
WARN [launcher]: Can not load "firefox", it is not registered!
  Perhaps you are missing some plugin?
```