[MERGE #1686 @MikeHolman] require min of VS2015 Update 3 for constexpr
[MERGE #1686 @MikeHolman] require min of VS2015 Update 3 for constexpr

Merge pull request #1686 from MikeHolman:vsconstexpr

mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore