Update requirements.txt to require gevent 0.13.1
Update requirements.txt to require gevent 0.13.1

This properly reflects the new standard set in
f685ae38e5ab590d68c5895883aeabd47c43dfbf, on discovering 0.13.0
probably never worked for anyone anyway after setting up tox.ini to
exercise this..

Conflicts:
	requirements.txt

nagual13/wal-e,alex/wal-e,modulexcite/wal-e,intoximeters/wal-e,DataDog/wal-e,wal-e/wal-e,RichardKnop/wal-e,equa/wal-e,fdr/wal-e,x86Labs/wal-e,tenstartups/wal-e,ArtemZ/wal-e,heroku/wal-e,ajmarks/wal-e