A while back a change in the travis R image made it necessary to add devtools here. (See for example FeatureExtraction).
Once this is fixed, a new version will be pushed to drat every time a DESCRIPTION file is pushed to master with a higher version number than the latest git tag. At the same time, a new git tag is created. (To get there, the build must pass R check)
Note that for R packages, the practice is to maintain release notes in a NEWS.md file, like here.
Also note that a best practice is to not allow any warnings in R check. i recommend removing this line, and of course fixing the warnings that currently exist.