phpunit: Fix failing DeprecatedGlobalTest on HHVM at Travis CI

phpunit: Fix failing DeprecatedGlobalTest on HHVM at Travis CI

Follows-up 1d77070f19e3. On Travis CI, this test fails on HHVM:

> There was 1 failure:
> 1) DeprecatedGlobalTest::testWarning
> Failed asserting that exception of type "PHPUnit_Framework_Error" is thrown.

I've been unable to reproduce this locally, but within Travis CI
it happens on both Trusty containers and Trusty VMs, no matter the
version of HHVM. After some trial and error, I narrowed it down
to the error_reporting setting, which presumably has a different default
in the HHVM package Travis CI uses.

Change-Id: I588218103d6c72df5ad4fb6289df688b3740233e