2.1.5 Bump
Add `viewable` field to Concept preserialize template

This bug was originally seen in cbmi/avocado#155. The client evaluates the
presence of `viewable` or `formatter_name` to determine if it should appear
in the columns dialog. Because `viewable` was not present, concepts using
the default (falsy) formatter would not appear.
2.1.5 Release