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?
If you're sure the text is correct, please merge.