Fix safe_eval() of set literals

There was code to support set literals (on Python 2.7 and newer), but it
was buggy: SAFE_NODES.union() doesn't modify SAFE_NODES in place,
instead it returns a new set object that is then silently discarded.

I added a unit test and fixed the code.  I also changed the version
check to use sys.version_tuple instead of a string comparison, for
consistency with the subsequent Python 3.4 version check that I added in
the previous commit.

lib/ansible/template/safe_eval.py

@@ -67,8 +67,8 @@ def safe_eval(expr, locals={}, include_exceptions=False):
     )
 
     # AST node types were expanded after 2.6
-    if not sys.version.startswith('2.6'):
-        SAFE_NODES.union(
+    if sys.version_info[:2] >= (2, 7):
+        SAFE_NODES.update(
             set(
                 (ast.Set,)
             )