Fix bug in SpanSelector, introduced in commit #dd325759

The useblit attribute was being set after calling new_axes,
but new_axes needs that attribute.  By setting useblit before
calling new_axes, we set it based on the initial canvas; even
though new_axes might change to a different canvas, we assume
it will be the same type of canvas (same backend), so its
supports_blit attribute will not change.
Merge pull request #1982 from efiring/SpanSelector_blit_bug

Fix bug in SpanSelector, introduced in commit dd325759
Merge pull request #1984 from mdboom/rgb_to_hsv

colors.rgb_to_hsv does not work properly with array of int dtype
Merge pull request #1985 from mdboom/horizontal-histogram

horizontal histogramm doesn't work in 1.2 branch
Merge remote-tracking branch 'upstream/v1.2.x'

Conflicts:
	lib/matplotlib/axes.py
Go back to one ghostscript process per test
Fix some test images that weren't being run on Travis before
Run tests in parallel on Travis and in tox
Include all test files
Make the test function name match so that they can be pickled through multiprocessing
Don't call "show" from a test
Fix typo so correct gs executable will be run
Remove import of old util
Don't run tests in parallel on Travis
Use $(nproc) to determine number of processors, as discovered by @pelson.
Use cbook.mkdirs, since with multiprocessing tests each process may try to create a directory at the same time
Make the log quieter
Only compare PNG version of hexbin extent -- it otherwise can crash inkscape on Travis