Test environments

OS X 10.13.3 (on travis-ci)
- current release (R 3.6.2)
ubuntu 16.04 (on travis-ci)
- R 3.2
- old release
- current release (R 3.6.2)
- development release
win-builder
- old release
- current release
- development release

R CMD check results

0 errors | 0 warnings | 0 notes