Don't look up interpreter if it doesn't contain a -

It turns out the underlying code errors pretty hard if the filename
doesn't contain a -. I haven't yet worked out why it's making the bad
lookup.

src/python/pants/backend/python/interpreter_cache.py

@@ -85,6 +85,8 @@ def select_interpreter_for_targets(self, targets):
 
   def _interpreter_from_path(self, path, filters):
     interpreter_dir = os.path.basename(path)
+    if '-' not in interpreter_dir:
+      return None
     identity = PythonIdentity.from_path(interpreter_dir)
     try:
       executable = os.readlink(os.path.join(path, 'python'))