Merge pull request #1506 from mdboom/testing/pytest-2.4.0-part3

Upgrade internal py.test to 2.4.0, and install that on Travis for good measure
Add a system whereby collections of units can be enabled after the fact.
Remove imperial units from examples
Fix documentation typo
Add astropy.units.imperial.enable
Add additional units in CDS/Vizier format
Move % and --- to the cds.py unit collection
Make Crab completely CDS-specific