Fix asdf extension url mapping bug

[jdavies-st]

Fixes #465.

Also:

• Moves the asdf pytest schema tester into the top-level conftest.py. pytest>=4 requires that plugins be specified only at the top level. This will re-enable the testing of ASDF schemas.
• Will use the schema tester via entry_points if available (currently in dev in asdf).

[nden]

This was failing with asdf on master, right? Could you add scipy to the Travis job testing with DEV versions. All DEV tests are currently failing because scipy is missing.

[jdavies-st]

Updated TravisCI dev matrix.

This wasn't failing on asdf master. It was interfering with asdf mappings. I've captured that behavior in the included test so it won't happen again.

[nmearl]

This looks really great, @jdavies-st! Thank you! Can you squash those last two commits? Then I think it's ready to merge in!

[jdavies-st]

Squashed/rebased. Looks like the allowed failure on Travis with dev versions of asdf and gwcs is still failing. I will throw some commits at it.

[nden]

The dev failures (or some of them) are resolved in #490. It's a chicken and egg. That one is failing because of the asdf problem. It's probably best to merge this one and then finish #490.

[jdavies-st]

I see. That makes sense.

Could someone kick the tires on the Travis build? Restart it?

Lots of problems with remote data and apt-get installs this week on Travis for some reason.

[nmearl]

This is looking good. @jdavies-st is this ready to merge now?

[jdavies-st]

Yes, merge away.