Allow manpage to build

This is a tiny change to allow the manpage to be built.

Manpage doesn't build because a require is missing from the Rakefile (it is part of ruby stdlib, but must be require-d explictily.)

This executes rake in the context of the bundle, so it would be more forgiving about the environment (and also warn about needed gems not being installed)

Also fixed the incorrect execute permission on the manfile.