Fix 'Configuring Bundler' links.

Now these point to the right version of the documentation for 'bundle config':
the command documentation for verions 1.2 and 1.3, and the HTML man page for
versions 1.0 and 1.1 (before the command page was complete).
Merge pull request #43 from TimMoore/fix-rationale-link-to-configuring

Fix 'Configuring Bundler' links.