Pin matplotlib to v1.3.1 to preserve test results.
Merge pull request #1293 from rhattersley/pin-matplotlib

Pin matplotlib to v1.3.1 to preserve test results.
BUG: Do not filter surface altitude/pressure fields
Merge pull request #1292 from cpelley/fixed_pp_filter

BUG: Do not filter surface altitude/pressure fields
Merge pull request #1295 from SciTools/v1.7.x

merging v1.7.x with master