Fix uninitialized constant error on --list

When executing memegen --list I got the following error:

```bash
/Users/gianu/work/memegen/lib/meme_generator/cli.rb:2:in `images': uninitialized constant MemeGenerator (NameError)
	from /Users/gianu/work/memegen/lib/meme_generator/cli.rb:11:in `list_generators'
	from /Users/gianu/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/memegen-1.0.9/bin/memegen:18:in `<top (required)>'
	from /Users/gianu/.rbenv/versions/2.0.0-p0/bin/memegen:23:in `load'
	from /Users/gianu/.rbenv/versions/2.0.0-p0/bin/memegen:23:in `<main>'
```

I'm using rbenv 0.4.0-22-gea3203d and ruby 2.0.0-p0.

The fix that I found (and it's working for me) is to add the require_relative in cli.rb.