Add Scribunto as SMW testextensions dependency

Without this dependency Scribunto_LuaEngineTestBase is an unknown class
and will fail the SMW\Test\PropertyLibraryTest [1]

[1] https://gerrit.wikimedia.org/r/#/c/56393/

Change-Id: I66c51ecb15f6e4dba82c9633aeda609a9adb1e3e