travis: Install ggplot from GitHub for now

jgabry · web-flow · commit 3922b87dd9cb · 2019-05-09T22:28:41.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -6,6 +6,7 @@ cache: packages
 
 r_github_packages:
   - r-lib/covr
+  - tidyverse/ggplot2
 
 r_build_args: '--no-build-vignettes'
 r_check_args: '--ignore-vignettes'
