Fix bug where add_loops assumes that edges are enumerated integers

[marcharper]

No description provided.

[drvinceknight]

Looks like another test has fallen over:

FAIL: test_init (axelrod.tests.unit.test_moran.TestMoranProcess)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/Axelrod-Python/Axelrod/axelrod/tests/unit/test_moran.py", line 50, in test_init
    [(0, 1), (2, 0), (1, 2), (0, 0), (1, 1), (2, 2)])
AssertionError: Lists differ: [(0, 1), (2, 0), (1, 2), (0, 0), (2, 2), (1, 1)] != [(0, 1), (2, 0), (1, 2), (0, 0), (1, 1), (2, 2)]
First differing element 4:
(2, 2)
(1, 1)
- [(0, 1), (2, 0), (1, 2), (0, 0), (2, 2), (1, 1)]
?                                        --------
+ [(0, 1), (2, 0), (1, 2), (0, 0), (1, 1), (2, 2)]
?                                  ++++++++

[drvinceknight]

Merging as it's a bug.