Update Travis config to include PHP 7.4 build

Also bump tested minor release branch to 3.7

.travis.yml

@@ -7,7 +7,7 @@ dist: trusty
 language: php
 
 env:
-  - DB=MYSQL CORE_RELEASE=3.6
+  - DB=MYSQL CORE_RELEASE=3.7
 
 matrix:
   include:
@@ -16,8 +16,12 @@ matrix:
     - php: 5.6
       env: DB=PGSQL CORE_RELEASE=3
     - php: 5.6
-      env: DB=PGSQL CORE_RELEASE=3.6
+      env: DB=PGSQL CORE_RELEASE=3.7
     - php: 7.1
+    - php: 7.4
+      dist: xenial
+      services:
+        - mysql
 
 before_script:
   - composer self-update || true