Revert "Test with serde on Travis. Fix #86"

This reverts commit a8f3529.

Nightly builds like https://travis-ci.org/servo/rust-cssparser/jobs/100079015
regularly get broken because on of serde’s dependencies needs to be updated
for changes in unstable Rust APIs.

This has so far never been an issue in cssparer itself, which only has
`#[derive]` attributes.

Author: SimonSapin

.travis.yml

@@ -10,7 +10,6 @@ script:
   - cargo test --verbose
   - cargo doc --verbose
   - ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --features heap_size)
-  - ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --features serde-serialization)
 
 after_success: |
   [ $TRAVIS_RUST_VERSION = nightly ] &&