try vignettes on travis again

Author: jgabry

.travis.yml

@@ -7,8 +7,8 @@ cache: packages
 r_github_packages:
   - r-lib/covr
 
-r_build_args: '--no-build-vignettes'
-r_check_args: '--ignore-vignettes'
+# r_build_args: '--no-build-vignettes'
+# r_check_args: '--ignore-vignettes'
 
 before_install:
   - mkdir -p ~/.R