Remove db default value from description in revisions

 Mysql gives this error when trying to provide a default value to text: ERROR 1101 (42000): BLOB, TEXT, GEOMETRY or JSON column
'description' can't have a default value.

We decided that it was better to have consistent behavior between
postgres and mysql than keep the default value for postgres.

Co-authored-by: Harsha Nandiwada <hnandiwada@vmware.com>
Co-authored-by: Merric de Launey <mdelauney@pivotal.io>

Author: Harsha Nandiwada

db/migrations/20200924180745_remove_default_revision_description.rb

@@ -0,0 +1,5 @@
+Sequel.migration do
+  change do
+    set_column_default(:revisions, :description, nil)
+  end
+end