Make WCS instances use WCSAxes if available
Added simple documentation
Added changelog entry [ci skip]
Added URL for more details on WCSAxes
Make comment into docstring as suggested by @mhvk
Warn on invalid WCS unit
CHANGES entry
Ensure that we don't install Python 2.7.9 on Travis (use 2.7.8 instead) otherwise we get certificate erros due to the changes to certificate verification in 2.7.9 (see PEP 476).
Merge pull request #3202 from astrofrog/fix-conda

Disable conda update since latest version breaks SSL certificate checks
Upgrade to wcslib 4.25
update setup_helpers and a corresponding setup.py change
Merge pull request #3204 from mdboom/wcs/upgrade-4_25

Upgrade to wcslib 4.25
skip doctest with matplotlib in skycoord for stability
Clarify what is returned
Merge pull request #3183 from astrofrog/wcsaxes-compatibility

WCSAxes compatibility hook
Merge pull request #3207 from hamogu/fix_3111

skip doctest with matplotlib in skycoord for stability
Remove workaround for Python 2.7.9 in .travis.yml
Merge pull request #3210 from astrofrog/remove-workaround

Remove workaround for Python 2.7.9 in .travis.yml
Merge pull request #3199 from mdboom/wcs/unit-warning

Warn on invalid WCS unit
Use union merge for changelog