Tidying Travis build

[goldingn]

Adding workaround for known bug in testthat

Switching to fork of R-builder to run with and without CRAN checks

[codecov-io]

Current coverage is 73.54%

Merging #197 into master will not affect coverage as of 323c6fd

@@            master    #197   diff @@
======================================
  Files           15      15       
  Stmts         1028    1028       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit            756     756       
  Partial          0       0       
  Missed         272     272       

Review entire Coverage Diff as of 323c6fd

Powered by Codecov. Updated on successful CI builds.