Use fbchat-asyncio 0.4.0a1 from PyPI

requirements.txt

@@ -5,4 +5,4 @@ ruamel.yaml>=0.15.94,<0.17
 commonmark>=0.8,<0.10
 python-magic>=0.4,<0.5
 mautrix==0.5.0.beta15
-https://github.com/tulir/fbchat-asyncio/tarball/rebase#egg=fbchat-asyncio
+fbchat-asyncio>=0.4.0.alpha1,<0.5