Update openpyxl dependency. Fixes #941

Just stumbled into the openpyxl requirements issue that @tommeagher points out in #941. It appears openpyxl has migrated to a [bitbucket team page](https://bitbucket.org/openpyxl/openpyxl). The [current repo](https://bitbucket.org/ericgazoni/openpyxl/) is now private and generates an auth password prompt during `pip` installation, causing the overall install to hang/fail at that point. The PR resolves issue by pointing to to the new public repo. 

This issue prevents a successful build of the stack, so might be worth a new patch release (1.1.2) and announcement on the PANDA group. I'm sending this PR to master since it appears you've already prepped it for such a patch release.