Upgrade to PHP 5.6 and PHPUnit 5.7/6.0

This patch updates the package to:

- Set the minimum supported PHP version to 5.6.
- Set the supported PHPUnit version to either 5.7 or 6.0, depending on
  PHP version used.

A number of test changes were required:

- All references to `PHPUnit_Framework_TestCase` were updated to the
  namespaced version. In any case where the class was not imported, an
  import statement was created.
- All `setExpectedException()` calls were updated to
  `expectException()`, and, if an expected message was provided,
  `expectExceptionMessage()`
- A number of `getMock()` calls still existed without usage of
  `getMockBuilder()`; these were updated.
- A zend-servicemanager test trait for testing container compatibility
  between zend-servicemanager versions differs from v2 to v3; the two
  instances in this particular suite were updated to override the
  necessary tests to vary strategy based on which PHP version was
  detected.
- New test failures were discovered in the flash messenger tests due to
  differences in how PHPUnit handles global state; these were corrected.

Author: weierophinney

composer.json

@@ -13,7 +13,7 @@
         }
     },
     "require": {
-        "php": "^5.5 || ^7.0",
+        "php": "^5.6 || ^7.0",
         "zendframework/zend-eventmanager": "^2.6.2 || ^3.0",
         "zendframework/zend-loader": "^2.5",
         "zendframework/zend-stdlib": "^2.7 || ^3.0"
@@ -40,7 +40,7 @@
         "zendframework/zend-session": "^2.6.2",
         "zendframework/zend-servicemanager": "^2.7.5 || ^3.0.3",
         "zendframework/zend-uri": "^2.5",
-        "phpunit/phpunit": "^4.6",
+        "phpunit/phpunit": "^5.7||^6.0",
         "zendframework/zend-coding-standard": "~1.0.0"
     },
     "suggest": {