Use github for dev installs

The old command doesn't work as 'dev' version isn't found:

````bash
$ pip install django-crispy-forms==dev
Downloading/unpacking django-crispy-forms==dev
  Could not find a version that satisfies the requirement django-crispy-forms==dev (from versions: 1.2.4, 1.0.0, 1.1.1, 1.1.2, 1.2.7, 1.1.4, 1.1.3, 1.2.0, 1.2.6, 1.2.1, 1.2.5, 1.2.2, 1.3.0, 1.3.2, 1.2.3, 1.3.1, 1.2.8)
No distributions matching the version for django-crispy-forms==dev
````