Return a value from value() when exception is thrown, to avoid compiler error
Merge branch 'n3672'
Remove constexpr from && version of value_or()

As per 20.5.4.5 of N3672, the && version of value_or() is not constexpr.
Enable rvalue references for *this in GCC 4.8.1+
Fix moved_on_value_or test

Change std::emplace to std::in_place
fix test for gcc 4.8.1+ so it will accept 5.0