Add lower bound for python in setup.

This ensures that if you pip install axelrod with an unsupported verion
of python that you get an error message:

```
(test-axelrod-on-3.4) ~: pip install ~/src/Axelrod
Processing ./src/Axelrod
Axelrod requires Python '>=3.5' but the running Python is 3.4.5
```

Author: drvinceknight

setup.py

@@ -29,5 +29,5 @@
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3 :: Only',
         ],
-    python_requires='>=3.0',
+    python_requires='>=3.5',
 )