Upgrade transifex-client dependency to 0.11

Version 0.11b3's setup.py had an undeclared dependency on `py2exe` when
running on Windows (see
https://github.com/transifex/transifex-client/blob/0.11b3/setup.py#L23),
which made it impossible to install without having a working `py2exe` to
begin with.

This has been fixed in 0.11 proper.

requirements/requirements-packaging.txt

@@ -5,4 +5,4 @@ wheel==0.24.0
 twine==1.4.0
 
 # Transifex client for managing translation resources.
-transifex-client==0.11b3
+transifex-client==0.11