Run coverage in parallel with tests

[eddelbuettel]

Given a test matrix, covr() can run while tests are running reducing overall time.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[codecov-io]

Codecov Report

Merging #1057 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1057   +/-   ##
=======================================
  Coverage   95.60%   95.60%           
=======================================
  Files          64       64           
  Lines        2778     2778           
=======================================
  Hits         2656     2656           
  Misses        122      122           

[eddelbuettel]

Just rearranging some more deckchairs in the .travis.yml file. No code change. Can be merged anytime.