Added the dependency requirement that yarl < 1.2

[sayeghr]

This is to prevent an aiohttp error with a breaking change in yarl 1.2.

Resolves #14

[sayeghr]

My understanding is that usually you want your requirements.txt file to be pinned, as thats what you'll be using during your development work and you'd want all contributors using the same version while developing, but the setup.py would have ranges, to accommodate users using your library having various versions of dependencies that the library supports.

I can change the requirements.txt to use <1.2 if you prefer.

[sayeghr]

See https://blog.miguelgrinberg.com/post/the-package-dependency-blues