add phpunit 4.8 to require-dev and force travisci to use local phpunit

Author: Shaun Bramley

.travis.yml

@@ -10,4 +10,4 @@ php:
 install: ./travis-init.sh
 
 script:
-  - phpunit --coverage-text
+  - ./vendor/bin/phpunit --coverage-text

composer.json

@@ -6,6 +6,9 @@
     "require": {
         "php": ">=5.4.0"
     },
+    "require-dev": {
+        "phpunit/phpunit": "~4.8"
+    },
     "suggest": {
         "ext-libevent": ">=0.1.0",
         "ext-event": "~1.0",