Allow for using the polyfills on PHP 5.4

[jrfnl]

There was only one PHP 5.5 specific syntax being used in this library - ::class -.
This was the only blocker for allowing tests using the polyfills to run on PHP 5.4.

This blocker has now been removed, making the polyfills available on PHP 5.4 in combination with PHPUnit 4.8.x as well.

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Note: there are 11 tests being marked as "risky" on PHP 5.4. This is the same on PHP 5.5 and is unrelated to this library, but to do with an underlying issue in the older PHPUnit versions themselves.
I do not consider this a blocker as the tests *will run and do pass.

Details of the tests marked as risky (fold out for details)

1) Yoast\PHPUnitPolyfills\Tests\Polyfills\ExpectExceptionTest::testExpectExceptionCodeException
Test code or tested code did not (only) close its own output buffers

2) Yoast\PHPUnitPolyfills\Tests\Polyfills\ExpectExceptionTest::testExpectExceptionMessageException
Test code or tested code did not (only) close its own output buffers

3) Yoast\PHPUnitPolyfills\Tests\Polyfills\ExpectExceptionTest::testExpectExceptionMessageAndCodeFailOnCode
Test code or tested code did not (only) close its own output buffers

4) Yoast\PHPUnitPolyfills\Tests\Polyfills\ExpectExceptionTest::testExpectExceptionMessageAndCodeFailOnMsg
Test code or tested code did not (only) close its own output buffers

5) Yoast\PHPUnitPolyfills\Tests\Polyfills\ExpectExceptionTest::testExpectExceptionMessageRegExpAndCodeFailOnCode
Test code or tested code did not (only) close its own output buffers

6) Yoast\PHPUnitPolyfills\Tests\Polyfills\ExpectExceptionTest::testExpectExceptionMessageRegExpAndCodeFailOnMsg
Test code or tested code did not (only) close its own output buffers

7) Yoast\PHPUnitPolyfills\Tests\TestListeners\TestListenerTest::testError
Test code or tested code did not (only) close its own output buffers

8) Yoast\PHPUnitPolyfills\Tests\TestListeners\TestListenerTest::testFailure
Test code or tested code did not (only) close its own output buffers

9) Yoast\PHPUnitPolyfills\Tests\TestListeners\TestListenerTest::testIncomplete
Test code or tested code did not (only) close its own output buffers

10) Yoast\PHPUnitPolyfills\Tests\TestListeners\TestListenerTest::testSkipped
Test code or tested code did not (only) close its own output buffers

11) Yoast\PHPUnitPolyfills\Tests\TestListeners\TestListenerTest::testStartStop
Test code or tested code did not (only) close its own output buffers

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