Pin lit to Python2-compatible version

Python2 testing of LNT fails because latest LIT no longer supports
Python2. This commit pins lit to an earlier version that still does.

Reviewed By: PrzemekWirkus

Differential Revision: https://reviews.llvm.org/D104705

Author: Thomas Preud'homme

setup.py

@@ -134,7 +134,7 @@
         "pyyaml==3.13",
         "requests",
         "future",
-        "lit",
+        "lit==0.11.1",
     ],
 
     ext_modules=[cPerf],