removing explicit reference to std::regex

[aaalgo]

Compile failed on old compilers due to explicit reference to std::regex.

My fork also has a change to move the hash function out of the templated class. If this is not desirable, please just fix REGEX_NS and disregard this PR.

[eidheim]

Thank you, though I need the case_insensitive_equals and case_insensitive_hash to be non-templated classes in order to be able to move headers to for instance Simple-WebSocket-Server without performing copy on a connection upgrade.

Regarding the REGEX_NS, I added https://github.com/eidheim/Simple-Web-Server/commit/19627bbe6bdfc85c627b4f1f038f43ef0f6b704d (with you as author).