import bilby fails out-of-the-box due to matplotlib backend issues

To reproduce (on macOS):

python -m virtualenv bilbyenv
./bilbyenv/bin/python -m pip install bilby
./bilbyenv/bin/python -c "import bilby"

The error presents as

/Users/duncan/tmp/./bilbyenv/bin/../lib/python3.6/site.py:165: DeprecationWarning: 'U' mode is deprecated
  f = open(fullname, "rU")
13:10 bilby INFO    : Running bilby version: 0.3.3:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/bilby/__init__.py", line 21, in <module>
    from . import core, gw, hyper
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/bilby/core/__init__.py", line 2, in <module>
    from . import likelihood, prior, result, sampler, utils
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/bilby/core/result.py", line 6, in <module>
    import corner
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/corner/__init__.py", line 37, in <module>
    from .corner import corner, hist2d, quantile
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/corner/corner.py", line 7, in <module>
    import matplotlib.pyplot as pl
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/matplotlib/pyplot.py", line 2374, in <module>
    switch_backend(rcParams["backend"])
...
  File "/Users/duncan/tmp/bilbyenv/lib/python3.6/site-packages/matplotlib/backends/qt_compat.py", line 165, in <module>
    raise ImportError("Failed to import any qt binding")
ImportError: Failed to import any qt binding

I think the fix is to update bilby.core.results to either call matplotlib.use('agg') before import matplotlib.pyplot:

diff --git a/bilby/core/result.py b/bilby/core/result.py
index df4374b..ac55d84 100644
--- a/bilby/core/result.py
+++ b/bilby/core/result.py
@@ -1,12 +1,16 @@
 import os
 from distutils.version import LooseVersion
+from collections import OrderedDict, namedtuple
+
 import numpy as np
 import deepdish
 import pandas as pd
-import corner
+
 import matplotlib
+matplotlib.use('agg')
+
+import corner
 import matplotlib.pyplot as plt
-from collections import OrderedDict, namedtuple

 from . import utils
 from .utils import logger, infer_parameters_from_function

I can happily post this patch as a merge request.