Add pytest-rerunfailures

Intermittent test failures - see:
matplotlib/matplotlib#7911.  Install
pytest-rerunfailures to trigger rebuild on failure.

Author: matthew-brett

.travis.yml

@@ -145,7 +145,7 @@ install:
     - python rm_test_images.py wheelhouse/*.whl
 
 script:
-    - TEST_DEPENDS="numpy==$NP_TEST_DEP pytest mock sphinx pytest pytest-timeout pytest-xdist pytest-faulthandler"
+    - TEST_DEPENDS="numpy==$NP_TEST_DEP pytest mock sphinx pytest pytest-timeout pytest-xdist pytest-faulthandler pytest-rerunfailures"
     # increase number of open files allowed for tests, on OSX
     # https://github.com/matplotlib/matplotlib/issues/3315
     - ulimit -n 4096