Update whatsnew v171
Removed spurious line from whatsnew
Merge pull request #1246 from dkillick/update_whatsnew_v1-7-1

Update whatsnew doc for v1.7.1
Bump version to 1.7.1-rc.1
Bump version to 1.7.1-rc.1.DEV
Clarify differentiate() coordinate values.
Remove obsolete image results
Merge pull request #1261 from rhattersley/obsolete-images

Remove obsolete image results
Allow for variance in netCDF4 behaviour.
Merge pull request #1260 from rhattersley/netcdf-masking

Allow for variance in netCDF4 behaviour.
Switch to miniconda test environment.
Update image results for later freetype.
Move cfchecker into conda env
Merge pull request #1262 from rhattersley/miniconda

Switch Travis environment to miniconda.
Bump version to 1.7.1
Bump version to 1.7.2-DEV
Add skip_data for tests that require it.
Merge pull request #1263 from QuLogic/tests-skip_data

Add skip_data for tests that require it.
Merge pull request #1253 from rhattersley/differentiate-docs

Clarify differentiate() coordinate values.