Remove Behat-related code dependant on PHPUnit 3.x

We've upgraded SilverStripe core to PHPUnit 4.x now, and the Functions.php has moved locations.
Since all custom contexts either use one of these contexts, or extend off SilverStripeContext,
we can be certain that the required assertions are automatically included for custom contexts,
no need to duplicate the require_once() calls.

Note that Behat 3.x suggests using static methods on a class, e.g. PHPUnit_Framework_Assert::assertSame (see http://docs.behat.org/en/master/quick_intro.html).
This seems quite verbose, and increases the upgrade burden on any project using Behat.

src/SilverStripe/BehatExtension/Console/Processor/InitProcessor.php

@@ -153,10 +153,6 @@ use SilverStripe\BehatExtension\Context\SilverStripeContext,
     SilverStripe\Framework\Test\Behaviour\CmsUiContext,
     SilverStripe\Cms\Test\Behaviour;
 
-// PHPUnit
-require_once 'PHPUnit/Autoload.php';
-require_once 'PHPUnit/Framework/Assert/Functions.php';
-
 /**
  * Features context
  *

src/SilverStripe/BehatExtension/Context/BasicContext.php

@@ -13,8 +13,7 @@ use Behat\Gherkin\Node\PyStringNode,
     Behat\Gherkin\Node\TableNode;
 
 // PHPUnit
-require_once 'PHPUnit/Autoload.php';
-require_once 'PHPUnit/Framework/Assert/Functions.php';
+require_once BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php';
 
 /**
  * BasicContext

src/SilverStripe/BehatExtension/Context/EmailContext.php

@@ -14,8 +14,7 @@ Behat\Gherkin\Node\TableNode;
 use Symfony\Component\DomCrawler\Crawler;
 
 // PHPUnit
-require_once 'PHPUnit/Autoload.php';
-require_once 'PHPUnit/Framework/Assert/Functions.php';
+require_once BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php';
 
 /**
  * Context used to define steps related to email sending.

src/SilverStripe/BehatExtension/Context/FixtureContext.php

@@ -9,8 +9,7 @@ use Behat\Behat\Context\BehatContext,
 	SilverStripe\Filesystem\Storage\AssetStore;
 
 // PHPUnit
-require_once 'PHPUnit/Autoload.php';
-require_once 'PHPUnit/Framework/Assert/Functions.php';
+require_once BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php';
 
 /**
  * Context used to create fixtures in the SilverStripe ORM.

src/SilverStripe/BehatExtension/Context/LoginContext.php

@@ -11,8 +11,7 @@ use Behat\Gherkin\Node\PyStringNode,
 Behat\Gherkin\Node\TableNode;
 
 // PHPUnit
-require_once 'PHPUnit/Autoload.php';
-require_once 'PHPUnit/Framework/Assert/Functions.php';
+require_once BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php';
 
 /**
  * LoginContext

src/SilverStripe/BehatExtension/Context/SilverStripeContext.php

@@ -20,6 +20,8 @@ use Symfony\Component\Yaml\Yaml;
 // Mink etc.
 require_once 'vendor/autoload.php';
 
+require_once BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php';
+
 /**
  * SilverStripeContext
  *