(travis) build on php7.3

.travis.yml

@@ -5,11 +5,11 @@ services:
   - docker
 
 env:
-  - V8VER=7.5
-    PHPVER=7.0
   - V8VER=7.5
     PHPVER=7.1
   - V8VER=7.5
     PHPVER=7.2
+  - V8VER=7.5
+    PHPVER=7.3
 
 script: make -f Makefile.travis test