Upgrade to latest version of mocha

This fixes an error that suddenly started appearing on travis builds
today:

    Error: global leak detected: documents

I'm not sure why upgrading fixes it, but all of the tests still
pass so it doesn't seem to have any negative consequences.
Store each translation as a separate key in elasticsearch

Rather than storing language objects in elasticsearch we simply flatten
out any language objects so they take the form '{field}_{language}'.