Remove with-serde in favor of just 'serde'

https://rust-lang-nursery.github.io/api-guidelines/naming.html#feature-names-are-free-of-placeholder-words-c-feature

.travis.yml

@@ -11,7 +11,7 @@ env:
 matrix:
   include:
     - rust: nightly
-      env: FEATURES=ipv6-iterator,ipv6-methods,with-serde
+      env: FEATURES=ipv6-iterator,ipv6-methods,serde,serde-derive
 
 script:
   - cargo build --features $FEATURES --verbose