CLI: Always require 'middleman-core/version'

The `init` command will fail if the `VERSION` was not required because
it is needed for the Gemfile generation. Unfortunately this was not
catched by the specs because the specs require the version and the child
process is not correctly reset.

I was not able to change this easily :-(
Merge pull request #1251 from railsbros-dirk/fix-middleman-init-command

CLI: Always require 'middleman-core/version'