Ignore .bundle and vendor per official Jekyll documentation

Per the [Jekyll documentation](https://jekyllrb.com/tutorials/using-jekyll-with-bundler/), you should add the `vendor/` and `.bundle/` paths to the .gitignore file, so you are not managing all dependencies within your repo.

Jekyll.gitignore

@@ -2,3 +2,6 @@ _site/
 .sass-cache/
 .jekyll-cache/
 .jekyll-metadata
+# Ignore folders generated by Bundler
+.bundle/
+vendor/