Add PHPUnit to dev dependencies

The easier it is for people to contribute, the better. Relying on a
global install of PHPUnit via PEAR doesn't sit well with some
developers. As other dev dependencies are installed via composer, why
not add PHPUnit itself to the list?

.travis.yml

@@ -9,4 +9,4 @@ php:
 before_script:
   - composer install --dev --prefer-source
 
-script: phpunit
+script: ./vendor/bin/phpunit

composer.json

@@ -17,6 +17,7 @@
         "psr/log": "~1.0"
     },
     "require-dev": {
+        "phpunit/phpunit": "~3.7.0",
         "mlehner/gelf-php": "1.0.*",
         "raven/raven": "0.5.*",
         "doctrine/couchdb": "dev-master"