upgrade compiler to C++17

[Vladimir Venediktov]

• see if travis can handle C++17 either gcc 7.1 or clang 5.0
• move our C++17 implementation of audit_buffer.hpp , audit_codec.hpp into official build
• change code as needed to compile on gcc >= 7.1 and on clang >= 5.0
• update README.md

[Vladimir Venediktov]

@mrbald well done!

[Vlad Lysyy]

all done, source code syntax changes kept to the minimum - only things that did not compile were fixed, further improvements are better to be done as separate focused changes.

it was fun :-)