Upgrade Travis build and minimum dependency versions

PHP 5.4+ can use phpunit v4.8.26 without generating any warnings.
HHVM cannot use phpunit v4.8.26
HHVM CAN use phpunit v5.4.6, but produces deprecated warnings - warnings only that are resolvable and do not interfere with the build or code coverage.

To achieve this, a build matrix is used such that PHP uses --prefer-lowest (which is phpunit v4.8.26) and HHVM uses the most recent version (phpunit v5.4.6 at this moment in time).

.travis.yml

@@ -12,9 +12,20 @@ php:
   - 7
   - hhvm
 
+env:
+  - COMPOSER_FLAGS=--prefer-lowest
+
+matrix:
+  exclude:
+    - php: hhvm
+      env: COMPOSER_FLAGS=--prefer-lowest
+  include:
+    - php: hhvm
+      env: COMPOSER_FLAGS=
+
 before_script:
     - travis_retry composer self-update
-    - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-source --prefer-stable
+    - travis_retry composer update --no-interaction --prefer-source --prefer-stable ${COMPOSER_FLAGS}
     - mysql -e 'create database phinx_testing;'
     - psql -c 'create database phinx_testing;' -U postgres
 

composer.json

@@ -27,7 +27,7 @@
         "symfony/yaml": "~2.8|~3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.0|^5.0"
+        "phpunit/phpunit": "^4.8.26|^5.0"
     },
     "autoload": {
         "psr-4": {