Build docs without optional impls

There's no way to identify them as opt-in in the docs which is a
constant source of confusion so it's safer to just not mention them
there.

Author: sfackler

.travis.yml

@@ -6,7 +6,7 @@ before_script:
 script:
 - cargo test
 - cargo test --features "uuid rustc-serialize time unix_socket serde"
-- cargo doc --no-deps --features "uuid rustc-serialize time unix_socket serde"
+- cargo doc --no-deps --features "unix_socket"
 after_success:
   - test $TRAVIS_PULL_REQUEST == "false" && test $TRAVIS_BRANCH == "master" && ./.travis/update_docs.sh
 env: