removed constexpr to support older VC compilers.

[joe2018Outlookcom]

In VS2013.5, the following statement causes a compilation error.

Notice that the same compatibility is done here.

RxCpp/Rx/v2/src/rxcpp/rx-util.hpp

Lines 67 to 72 in f38fb8e

	// removed constexpr & noexcept to support older VC compilers
	template<typename T>
	/*constexpr*/ T const &as_const(T & t) /*noexcept*/
	{
	return t;
	}