Fix build with latest rebar
Change Erlson syntax to use brackets instead of curly braces

This change is backward compatible. The old syntax is still supported with
Erlang R16 and earlier releases.

Erlang 17 introduced a new built-in `maps` type. Maps syntax conflicts with the
original Erlson syntax that relied on curly braces.

For this reason, we changed Erlson syntax to use brackets instead. Such change
eliminates grammar conflicts and makes Erlson syntax look sufficiently different
from maps.
Add Erlang 17 to Travis-CI build matrix