gemspec: require minigit 0.0.4

Before this commit:

* on fresh bundle install, bundler installs minigit-0.0.4
* on existing bundle, bundler does not automatically require 0.0.4

After this commit: bundler forces author to use minigit-0.0.4 to
ensure all users have a consistent version.

Signed-off-by: Paul Morgan <jumanjiman@gmail.com>