build: Drop PHP 5 from Travis matrix

We already cover this in our own Jenkins CI, and apparently
the PHP 5.5+mysql build recently started failing with a
segmentation fault for unknown reasons.

Change-Id: Ie041d5378155bb9ddcbf413f546a3a04efcb4b4f

.travis.yml

@@ -23,22 +23,20 @@ matrix:
     # On Trusty, mysql user 'travis' doesn't have create database rights
     # Postgres has no user called 'root'.
     - env: dbtype=mysql dbuser=root
-      php: 5.5
+      php: 7.2
+    - env: dbtype=mysql dbuser=root
+      php: 7.1
     - env: dbtype=postgres dbuser=travis
-      php: 5.5
+      php: 7.1
+    - env: dbtype=mysql dbuser=root
+      php: 7
     # https://docs.travis-ci.com/user/languages/php#HHVM-versions
-    - env: dbtype=mysql dbuser=root
-      php: hhvm-3.18
-    - env: dbtype=mysql dbuser=root
-      php: hhvm-3.21
     - env: dbtype=mysql dbuser=root
       php: hhvm-3.24
     - env: dbtype=mysql dbuser=root
-      php: 7
+      php: hhvm-3.21
     - env: dbtype=mysql dbuser=root
-      php: 7.1
-    - env: dbtype=mysql dbuser=root
-      php: 7.2
+      php: hhvm-3.18
 
 services:
   - mysql