Update Travis-CI to R

[frenchja]

By updating to https://docs.travis-ci.com/user/languages/r, which supports the installation of binary R packages from https://launchpad.net/~marutter/+archive/ubuntu/c2d4u, we can potentially speed up testing and build time.

[coveralls]

Coverage remained the same at 95.628% when pulling bbdc157 on update_travis into a973b9c on master.

[frenchja]

Missing link or links in documentation object 'dependency-tracking.Rd':

  ‘active_resource’ ‘resource_caching’

Missing link or links in documentation object 'director.Rd':

  ‘https://github.com/wch/R6’

Missing link or links in documentation object 'modification-tracking.Rd':

  ‘active_resource’

Missing link or links in documentation object 'parser.Rd':

  ‘active_resource’

Missing link or links in documentation object 'preprocessor.Rd':

  ‘active_resource’

Missing link or links in documentation object 'resource-caching.Rd':

  ‘active_resource’ ‘dependency_tracking’

Missing link or links in documentation object 'resource.Rd':

  ‘directorResource’

Missing link or links in documentation object 'virtual-resource.Rd':

  ‘active_resource’

@robertzk

[robertzk]

@frenchja Let's revisit this next hackathon.