[tests] Fix expected exception message on wrong key type.

Redis >= 2.8 returns -WRONGTYPE errors instead of -ERR when executing
operations on wrong key type (such as trying to LPUSH on a string key).

Luckily for us, phpunit's @expectedExceptionMessage annotation actually
does not perform an exact match but works on a substring so we just omit
the initial part of the exception message to make the test work.

tests/Predis/Command/StringBitOpTest.php

@@ -168,7 +168,7 @@ class StringBitOpTest extends CommandTestCase
     /**
      * @group connected
      * @expectedException Predis\ServerException
-     * @expectedExceptionMessage ERR Operation against a key holding the wrong kind of value
+     * @expectedExceptionMessage Operation against a key holding the wrong kind of value
      */
     public function testThrowsExceptionOnInvalidSourceKey()
     {