Ignore PendingDeprecationWarning from Coverage

`PendingDeprecationWarning`s are coming from Coverage.py, specifically `import coverage` on line 348 of tester.py: 

```
/usr/lib/python3.4/imp.py:32: PendingDeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  PendingDeprecationWarning)
```

https://github.com/python-imaging/Pillow/blob/master/Tests/tester.py#L348

Coverage.py, execfile.py, line 3, `import imp`:

https://github.com/nedbat/coveragepy/blob/master/coverage/execfile.py#L3

Let's ignore the warning from Coveralls for now so we can have Python 3.4 as a must-pass.

This resolves https://github.com/python-imaging/Pillow/issues/592.

I've [reported the warning to Coverage.py](https://bitbucket.org/ned/coveragepy/issue/305/pendingdeprecationwarning-the-imp-module).