Update .travis.yml for cargo doc

.travis.yml

@@ -17,3 +17,11 @@ script:
 - cargo build --verbose
 - cargo test --verbose
 - cargo test --manifest-path=spec/Cargo.toml
+after_success: |
+  [ $TRAVIS_BRANCH = master ] &&
+  [ $TRAVIS_PULL_REQUEST = false ] &&
+  cargo doc &&
+  echo "<meta http-equiv=refresh content=0;url=`echo $TRAVIS_REPO_SLUG | cut -d '/' -f 2`/index.html>" > target/doc/index.html &&
+  sudo pip install ghp-import &&
+  ghp-import -n target/doc &&
+  git push -fq https://${GH_TOKEN}@github.com/${TRAVIS_REPO_SLUG}.git gh-pages