Fix for BOOST_NO_CXX11_RVALUE_REFERENCES

Fix compilation of future.hpp with BOOST_NO_CXX11_RVALUE_REFERENCES defined. Tested on clang and MSVC12 with and without BOOST_NO_CXX11_RVALUE_REFERENCES defined.

BOOST_NO_CXX11_RVALUE_REFERENCES has to be defined if the compiler provides rvalue references, but the standard library doesn't provide std::move. Without these fixes, the deleted copy constructor is called.