Explicitly require digest/md5

When using Ruby 2.0.0-p353, I was getting this error after editing a file using poet:

```bash
 ~ $ poet edit private
/Users/mateusz/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/poet-0.8/lib/poet.rb:111:in `edit': uninitialized constant PoetCLI::Digest (NameError)
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/poet-0.8/bin/poet:5:in `<top (required)>'
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/bin/poet:23:in `load'
	from /Users/mateusz/.rbenv/versions/2.0.0-p353/bin/poet:23:in `<main>'
```

Including the `Digest::MD5` module explicitly did fix it. The problem is I could not reproduce this behavior in the test suite, probably because of the module being required by some other gems by coincidence.