Avoid deprecation warnings in test suite due to deprecation of getMock() in PHPUnit

[ghost]

The method getMock() was deprecated with the release of PHPUnit 5.4. The same release introduced createMock() that should be used to create mock objects instead. This PR modifies React\Tests\EventLoop\TestCase::createCallableMock() to use the new mock API if available.

[jsor]

We've currently decided to add PHPUnit 4.8 to require-dev of each component, see reactphp/reactphp#354. This will fix this issue.

As a side note: i've also fixed this latetly in react/promise by using

$this
->getMockBuilder('React\Promise\Stub\CallableStub')
->getMock();

This works in all version of PHPUnit, see reactphp/promise@73f2304#diff-d4c8c6dc8769324fc27cfdac19f05caf

[ghost]

Should i change the PR to use mock builder or close it and let you introduce PHPUnit 4.8 instead?

[jsor]

I'd vote for getting this in for forward compatibility and to avoid warnings when users run the tests not from vendors but with their local version of PHPUnit.

[clue]

Thanks @martinschroeder and @jsor!