Fix docs Rake "update_version" command to strip trailing spaces

[zpao]

Looks like it's mostly a problem with Ruby 1.8 yaml vs 1.9 yaml. One strips the trailing space and the other leaves it in.

There's also the issue of reordering. So at the very least we shouldn't re-write _config.yml if the version is the same.

[zpao]

I changed this to only rewrite the file on version bumps in de2832c, so I'm going to call it good enough and close this out.