Reduce travis test to min and max php only

.travis.yml

@@ -37,7 +37,6 @@ php:
 - '7.0'
 - '7.1'
 - '7.2'
-#- hhvm		only with dist: trusty
 - nightly
 
 env:
@@ -63,6 +62,14 @@ matrix:
   - php: nightly
   # We exclude some combinations not usefull to save Travis CPU
   exclude:
+  - php: '5.5'
+    env: DB=mysql
+  - php: '5.6'
+    env: DB=mysql
+  - php: '7.0'
+    env: DB=mysql
+  - php: '7.1'
+    env: DB=mysql
   - php: '5.5'
     env: DB=postgresql
   - php: '5.6'
@@ -71,8 +78,6 @@ matrix:
     env: DB=postgresql
   - php: '7.1'
     env: DB=postgresql
-  - php: hhvm
-    env: DB=postgresql
   - php: nightly
     env: DB=postgresql