[TASK] Downgrade to PHPUnit 3.7.*

Suddenly build failures occurred on travis which seem
to be related to ReflectionClass::newInstanceWithoutConstructor
which is used by PHPUnit 4.1.* when available.

Downgrade to PHPUnit 3.7.* to have a green build again
as the failures are totally unrelated to our code.

Change-Id: I6c3c200d5eda372e92ee0fd4c39e7fe79183ecab
Reviewed-on: https://review.typo3.org/30616
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel