Need to install self to build vignette.

https://travis-ci.community/t/deployment-of-pages-with-r-travis-fails-with-pkgdown-when-vignettes-includes-the-current-package/2048/2

.travis.yml

@@ -11,5 +11,5 @@ addons:
 before_cache: Rscript -e 'remotes::install_cran("pkgdown")'
 deploy:
   provider: script
-  script: Rscript -e 'pkgdown::deploy_site_github()'
+  script: Rscript -e 'devtools::install(); pkgdown::deploy_site_github()'
   skip_cleanup: true