Update boost to 1.68.0

Keeps the boost libraries up to date. This also silences informational
messages that get spammed throughout the build, such as:

"Info: Boost.Config is older than your compiler version - probably
nothing bad will happen - but you may wish to look for an update Boost
version.  Define BOOST_CONFIG_SUPPRESS_OUTDATED_MESSAGE to suppress this
message."

Which makes the compilation process a lot less noisy on Windows. It's
now much easier to actually spot warnings that occur.

boost/range/algorithm_ext/insert.hpp

@@ -39,6 +39,7 @@ inline Container& insert( Container& on, const Range& from )
     BOOST_RANGE_CONCEPT_ASSERT(( ForwardRangeConcept<Container> ));
     BOOST_RANGE_CONCEPT_ASSERT(( SinglePassRangeConcept<Range> ));
     on.insert(boost::begin(from), boost::end(from));
+    return on;
 }
 
     } // namespace range