Pin pynacl to 0.3.0

Something has gone wrong in the packaging of 1.* which causes it not to
compile.

synapse/python_dependencies.py

@@ -22,7 +22,7 @@
     "unpaddedbase64>=1.0.1": ["unpaddedbase64>=1.0.1"],
     "canonicaljson>=1.0.0": ["canonicaljson>=1.0.0"],
     "signedjson>=1.0.0": ["signedjson>=1.0.0"],
-    "pynacl>=0.3.0": ["nacl>=0.3.0", "nacl.bindings"],
+    "pynacl==0.3.0": ["nacl==0.3.0", "nacl.bindings"],
     "service_identity>=1.0.0": ["service_identity>=1.0.0"],
     "Twisted>=15.1.0": ["twisted>=15.1.0"],
     "pyopenssl>=0.14": ["OpenSSL>=0.14"],