Rebuild old docs when the version list changes

[clintongormley]

When adding a new doc version, or removing an existing version, the existing docs need to be rebuilt in order to update the version selector.

[clintongormley]

Hmm, this is a deficiency in my docs build script. It doesn't detect that a new version has been added so it doesn't rebuild the old docs. Closing in favour of #59

[jsvd]

@clintongormley after clicking two times in "Closing in favour of #59" I should note that this issue was closed in favor of itself :)

[clintongormley]

Doh :D

[dedemorton]

This issue should be addressed as part of doc infra changes that are planned.