Fixed problem when a container has the name of an existing image

Hi,

I needed some rake tasks exactly like the ones you made.
There is no need to use a gemfile in my case as i don't want to bundle anything and needlessly comlicate things so i just get the code and require it.
This is why i changed in to require relative instead of require.
As far as i can tell it has no adverse effects.
Another problem i encountered is when a container had the name of an image.
"docker inspect" would return the details of that image and will cause the 'has_container?' method to return wrong true even though there is no container.
I hope you'll find these changes useful too.

Thanks for sharing your work!
Gal.