Update Ruby tooling to expect Bundler 2.3.7

This version fixes an issue that could have created confusion when
running `BUNDLE_WITH=screenshots bundle install` would result in an
automated change to `.bundle/config`.

See rubygems/rubygems#3708 (comment)

Gemfile.lock

@@ -341,4 +341,4 @@ DEPENDENCIES
   xcpretty-travis-formatter
 
 BUNDLED WITH
-   2.3.6
+   2.3.7