renamed LICENSE.txt to LICENSE

This is the best pull request Ghost has or will ever receive. Yes I have renamed the license file.

Joking aside am just trying to get feet wet with Ghost, hope to make real contributions soon as more familiar with the codebase.

Why did I remove the file extension? Well its the common convention with node projects.

By the way I did not run any tests. Don't think I needed for such a trivial change and I don't have the vagrant setup yet.