fix cargo test --doc

Signed-off-by: Yoshua Wuyts <[email protected]>

Author: yoshuawuyts

Diff for: .travis.yml

@@ -64,4 +64,4 @@ matrix:
 
 script:
   - cargo check --features unstable --all --benches --bins --examples --tests
-  - cargo test --all --docs --features unstable
+  - cargo test --all --doc --features unstable