Add preliminary PHP 7.4 testing (#151)

Add preliminary PHP 7.4 testing

Author: schlessera

.travis.yml

@@ -52,6 +52,9 @@ jobs:
         - composer lint
         - composer phpcs
       env: BUILD=sniff
+    - stage: test
+      php: 7.4snapshot
+      env: WP_VERSION=latest
     - stage: test
       php: 7.3
       env: WP_VERSION=latest
@@ -80,3 +83,7 @@ jobs:
     - stage: deploy
       env: DEPLOY_BRANCH=master
       script: ./ci/deploy.sh
+  allow_failures:
+    - stage: test
+      php: 7.4snapshot
+      env: WP_VERSION=latest