don't try to drop cassandra column in 1.2.x

test plan: run migrations in cassandra 1.2.x, and you shouldn't get an
error. migrations in 1.1 and 2.0 should still drop the
page_views.contributed column.

Change-Id: I18d0fc52c8a9f6e86e3f4907a0bf2e9a7aaee283
Reviewed-on: https://gerrit.instructure.com/36528
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>
Tested-by: Brian Palmer <brianp@instructure.com>