Merge pull request #86 from afk11/fix-travis-php-versions

test 7.3 & 7.4, nightly now refers to 8.0

Author: afk11

.travis.yml

@@ -4,7 +4,8 @@ php:
   - 7
   - 7.1
   - 7.2
-  - nightly
+  - 7.3
+  - 7.4snapshot
 
 dist: trusty