Temporarily downgrade phpunit/phpunit-mock-objects
Temporarily downgrade phpunit/phpunit-mock-objects

Version 2.3.1 uses a different reflection methodology which is currently broken in HHVM
(see https://github.com/facebook/hhvm/issues/5170).  Downgrading will allow our tests to pass while this
issue is resolved.

postalservice14/commonmark,kkiernan/commonmark,kkiernan/commonmark,fesor/commonmark,garethellis36/commonmark,thephpleague/commonmark,colinodell/commonmark,hason/commonmark,thephpleague/commonmark,fesor/commonmark,postalservice14/commonmark,garethellis36/commonmark,hason/commonmark