Don't run mdbook test

We don't want to test almost any of our code blocks... none of them are valid on their own...

Author: mark-i-m

.travis.yml

@@ -13,7 +13,6 @@ install:
 - bash -x ci/install.sh
 script:
 - mdbook build
-- mdbook test
 notifications:
   email:
     on_success: never