Update config page link to the recent version

In the `man/bundle-gem.ronn`, we are using a hard coded version
number to link the `bundle-config` doc, which is `v1.14` in this
case. This commit updates that link and use relative path so it
will always point to the recent version.

Ref: https://bundler.io/v1.16/man/bundle-gem.1.html

man/bundle-gem.ronn

@@ -75,4 +75,4 @@ configuration file using the following names:
 
 ## SEE ALSO
 
-* [bundle-config](http://bundler.io/v1.14/bundle_config.html)
+* [bundle-config](./bundle-config.1.html)