fix: need gevent >= 1.2, not > 1.1

Python 2.6 support was dropped in 1.2, so 2e8dcd3 simplified the dependency graph to require `gevent>1.1`. However, bugfix releases to the 1.1 series (1.1.1, etc) will satisfy this requirement, so we should actually be requiring `gevent >= 1.2`

requirements_gevent.txt

@@ -1 +1 @@
-gevent>1.1
+gevent>=1.2