Plato stack update to lts-16.26 finds an error with tuura-boolean.

[jrbeaumont]

It states that there is an ambiguity in Tuura.Boolean.

Line 43 uses <> which is ambiguous as to whether it belongs to Text.PrettyPrint.HughesPJClass or GHC.Base.

The branch plato-fix provides a fix for this, in an attempt to fix plato issue #97