Tell travis to use dox feature

Author: tmerr

.travis.yml

@@ -16,9 +16,9 @@ before_script:
 script:
 - |
   travis-cargo build &&
-  travis-cargo --only stable doc
+  travis-cargo --only nightly doc -- features dox
 after_success:
-- travis-cargo --only stable doc-upload
+- travis-cargo --only nightly doc-upload -- features dox
 env:
   global:
   # don't have special dependencies for nightly build