Bump the version of twisted needed for setup_requires to 15.2.1

setup.py

@@ -48,7 +48,7 @@ setup(
     description="Reference Synapse Home Server",
     install_requires=dependencies['requirements'](include_conditional=True).keys(),
     setup_requires=[
-        "Twisted==14.0.2", # Here to override setuptools_trial's dependency on Twisted>=2.4.0
+        "Twisted==15.2.1", # Here to override setuptools_trial's dependency on Twisted>=2.4.0
         "setuptools_trial",
         "mock"
     ],