Upgrade to CC BY 4.0

Address  #8.

## Step by Step

~~~
$ cd lesson-template
$ git format-patch -1 --stdout 7d018cca > /tmp/cc.patch
~~~

Edit `/tmp/cc.patch` to remove conflict due "Software Carpentry, Ltd" vs "Software Carpentry Foundation".

~~~
$ cd ../workshop-template
$ git am /tmp/cc.patch
~~~

Edit `LICENSE.md` to back changes due previous conflict.

~~~
$ git commit -a --amend
~~~

@wking Could you check that the commit is OK?
It looks like it's there to me:

![cc-by-4](https://cloud.githubusercontent.com/assets/209920/5515724/ff522698-882b-11e4-8b4f-7a9d7e7193ad.png)