boolector: add version 1.5 and 1.6

There are two versions here because beginning with 1.6.0, Boolector has
a more restrictive, unfree license which disallows commercial use.

As a result, Boolector 1.5 is the default 'boolector' expression.

Signed-off-by: Austin Seipp <aseipp@pobox.com>