changed rawgit to raw.githack for Travis builds

I saw some Travis builds break because rawgit.com is no longer serving 
content for some repositories. For all my repositories, I changed 
rawgit.com to raw.githack.com, which is currently working. This is 
normally for building the static editors' draft. I did this in the 
master branch only, so if you need this build in other branches you may 
need to pull the commit into them.

In the future it may make sense to parameterize this if we end 
frequently needing to rotate services like this. Or even better not 
depend on a web service and be able to generate the specs completely 
within the Travis build environment, which is supposed to be possible 
but I have not yet succeeded at making it work.

Michael