Update Travis builds to include PHP 7, update PHPUnit to 5.7

.travis.yml

@@ -1,19 +1,21 @@
 # See https://github.com/silverstripe/silverstripe-travis-support for setup details
 
-sudo: false
-
 language: php
 
 dist: trusty
 
 matrix:
   include:
-    - php: 5.6
-      env: DB=PGSQL CORE_RELEASE=3.5
-    - php: 7.0
-      env: DB=MYSQL CORE_RELEASE=3
-    - php: 7.1
-      env: DB=MYSQL CORE_RELEASE=3.6
+    - php: '5.6'
+      env: DB=PGSQL CORE_RELEASE=3.6
+    - php: '7.0'
+      env: DB=MYSQL CORE_RELEASE=3.7
+    - php: '7.1'
+      env: DB=MYSQL CORE_RELEASE=3.7
+    - php: '7.2'
+      env: DB=MYSQL CORE_RELEASE=3.7
+    - php: '7.3'
+      env: DB=MYSQL CORE_RELEASE=3.7
 
 before_script:
   - composer self-update || true

composer.json

@@ -20,6 +20,9 @@
     "require": {
         "silverstripe/framework": "3.*"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^5.7"
+    },
     "extra": [],
     "license": "BSD-3-Clause"
 }