v1.2.7 - Temporary constantinopleFix Release (5cef36e)

Updated tests submodule to 5cef36e (Jan 24, 2019) ethereum/tests commit
which merges the PR #582
with the test updates on the Constantinople pre-/post reentrancy fixes.

This is just a temporary release to get some basis for further development
and should be replaced with a tag-release based release once a new
tag-release becomes available on the ethereum/tests repo.