Replace iovec with std::io::IoSlice

- Renames `bytes_vec` to `bytes_vectored` and `bytes_vec_mut` to
  `bytes_vectored_mut`.

.travis.yml

@@ -3,7 +3,8 @@ dist: trusty
 language: rust
 services: docker
 sudo: required
-rust: stable
+#rust: stable
+rust: beta # we need 1.36, which is still beta
 
 env:
   global:
@@ -19,7 +20,7 @@ matrix:
     #
     # This job will also build and deploy the docs to gh-pages.
     - env: TARGET=x86_64-unknown-linux-gnu
-      rust: 1.28.0
+      #rust: 1.36.0 (not stable yet)
       after_success:
         - |
             pip install 'travis-cargo<0.2' --user &&