Enable builds for PHP 7.3

Author: sergeyklay

.travis.yml

@@ -9,6 +9,7 @@ php:
     - 7.0
     - 7.1
     - 7.2
+    - nightly
 
 git:
     depth: 1
@@ -34,7 +35,7 @@ env:
 matrix:
     fast_finish: true
     allow_failures:
-        - php: 7.2
+        - php: nightly
     exclude:
         - env: CC="gcc"
           compiler: clang