Adding TextClause support to server_default

Gist comparing autogenerated output from your version and my version of Alembic: https://gist.github.com/cerivera/5c01f8f2e0f2b282472e

(This was tested on PostgreSQL)

Your migration script failed to upgrade: https://gist.github.com/cerivera/bf567dedd80a46050c08

Table description after upgrading with my migration script: https://www.evernote.com/shard/s203/sh/a9e50835-7314-4b04-a3b8-923ac5e3a16a/ffff96f80ec5e45abff0c4b3dbf7a200
this is really fixing #197 at this point right  https://bitbucket.org/zzzeek/alembic/issue/197/support-rendering-of-funcxyz-funcxyz