Upgrade to API 31.0

This branch now passes all tests with a few adaptations to changes in 30.0 and 31.0.

It sets the <apiVersion> element in all meta.xml files to 31.0 to ensure consistency and includes changes to the updateMetaXml ant target to update all meta.xml files to api version specified in cumulusci.properties (cumulus.project.apiVersion).

I think we want to do more testing of this before merging.  I'm creating the pull request so we can use it to track discussion of what other testing we want to do and the results of those tests.