feat(kernel/error_msgs): expose pp_indent_expr

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/kernel_bindings): incorrect use of pushinteger, and improve justification Lua API

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/justification): bug in depends_on

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/justification): add is_eqp predicate

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add justification API tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(travis): use lean-build@googlegroups