Explicitly specify distribution for travis.
Explicitly specify distribution for travis.

Travis shifted the default distribution, but the yml file can't work on
trusty, xenial, andr bionic; use xenial explicitly.

Add Python 3.7 testing.

girder/large_image,girder/large_image,girder/large_image,DigitalSlideArchive/large_image,DigitalSlideArchive/large_image,DigitalSlideArchive/large_image