THRIFT-3515 Python 2.6 compatibility and test on CI

This closes #766
diff --git a/build/docker/centos6/scripts/keepit b/build/docker/centos6/scripts/keepit
new file mode 100644
index 0000000..cb885df
--- /dev/null
+++ b/build/docker/centos6/scripts/keepit
@@ -0,0 +1 @@
+keep it