Add php 7.3 (#31)

Travis CI updates:
* Add php 7.3
* Remove hhvm from build

.travis.yml

@@ -5,20 +5,15 @@ matrix:
   include:
     - php: 5.6
       env: deps=low
-    - php: hhvm
-      env: deps=low
     - php: 7.0
       env: deps=low
     - php: 5.6
-    - php: hhvm
     - php: 7.0
     - php: 7.1
     - php: 7.2
+    - php: 7.3
   fast_finish: true
 
-before_install:
-  - composer config --global --auth github-oauth.github.com $GITHUB_OAUTH_TOKEN
-
 install:
   - composer self-update
   - if [ "$deps" = "no" ]; then COMPOSER_MEMORY_LIMIT=-1 composer --prefer-stable --no-interaction update; fi;