[contract] new maintenance release for Boost 1.72

From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2019-10-07 16:00:34

Hello Release Managers & all,

I made a maintenance release of Boost.Contract. All tests pass, docs
are updated, etc. The code is up-to-date in boost/contract/master.

Can you please pull changes from boost/contract/master into the
upcoming Boost 1.72 release?

These are the changes:
* Using std::uncaught_exceptions on C++17 compilers onward (instead of
the now obsolete std::uncaught_exception).
* Fixed a few warnings.
* Removed linking to Boost.System (Boost.System is now a header-only library).
* Added this library to Boost's Continuous Integration (CI).
* Fixed documentation typos.

Thank you in advance.
--Lorenzo