Updates from release branch

[mdickinson]

Bring changes from the release branch back into master:

• Conditionally skip a test that requires TraitsUI (Test fix: skip test that requires Traits UI if Traits UI is not available. #467)
• Update changelog (Update changelog for 5.1.0 release. #465, Add missing changelog entries for #466 and #467. #468)
• Update Appveyor and Travis CI configurations to run tests for all PRs; not just those against master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@99fbb56). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #469   +/-   ##
=========================================
  Coverage          ?   65.26%           
=========================================
  Files             ?       44           
  Lines             ?     7057           
  Branches          ?     1411           
=========================================
  Hits              ?     4606           
  Misses            ?     2025           
  Partials          ?      426

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[mdickinson]

Self-merging, after sanity check; all these changes have already been reviewed on their way into the release branch.