Use `requests` instead of `Httplib2`, which has problems with SSL on som...

...e systems.

Apparently Httplib2 doesn't take into account system root certificates, and is patched to do so on several Linux distributions. Unfortunately it seems that when `Httplib2` is installed in a `virtualenv` on those systems, it is the un-patched version and you can get SSL certificate verification errors. The `requests` library doesn't seem to suffer from the same problems, so I have switched out `Httplib2` for `requests` in the `tastyapi.clients` module, where I was having problems.

More info: https://github.com/pennersr/django-allauth/issues/113