Define WEBWORK_DIRECTORY if it is not already defined.

Backported from develop
Fixes issue #16
Merge branch 'release/2.7' of https://github.com/openwebwork/webwork2 into release/2.7
Merge pull request #83 from mgage/release/2.7

Define Constants::WEBWORK_DIRECTORY from ENV{WEBWORK_ROOT} if it is not already defined.