Move requests_toolbelt to requirements.txt

[manu-chroma]

Since it is not a test requirement and is actually used in the username_api source:

https://github.com/manu-chroma/username-availability-checker/blob/master/username_api.py#L8

[seeeturtle]

@manu-chroma, the reason i did like that is because the result of it is actually used in test, not the server. you can know that from travis file.

[manu-chroma]

but you cannot run the server without installing test requirements. That's why it should be moved into main requirements

[seeeturtle]

Then I think it should be only used when testing, not putting in requirements.txt