Updated .travis.yml to perform CI against PHP 7.3 and nightly

.travis.yml

@@ -1,17 +1,30 @@
 language: php
+
 php:
   - 7.1
   - 7.2
+  - 7.3
+  - nightly
+
+matrix:
+  allow_failures:
+    - php:
+        - nightly
+
 install:
   - travis_retry composer self-update
   - travis_retry composer install --no-interaction --no-suggest
+
 script:
   - vendor/bin/phpunit --coverage-clover=coverage.xml
+
 after_script:
   - wget http://psvcg.coreteks.org/php-semver-checker-git.phar
   - php php-semver-checker-git.phar suggest -vvv
+
 after_success:
   - bash <(curl -s https://codecov.io/bash)
+
 cache:
   directories:
     - $HOME/.composer/cache