Ignore /vendor/bundle (#1880)

.gitignore

@@ -4,8 +4,9 @@
 # or operating system, you probably want to add a global ignore instead:
 #   git config --global core.excludesfile '~/.gitignore_global'
 
-# Ignore bundler config.
+# Ignore bundler config and downloaded libraries.
 /.bundle
+/vendor/bundle
 
 # Ignore the default SQLite database.
 /db/*.sqlite3