Removal of C++14-specific std::make_unique from mongo_sink.h

[jabartek]

mongo_sink.h used std::make_unique instead of spdlog::details::make_unique, thus making it impossible to compile a project under C++11 which spdlog itself targets. While mongocxx requires use of polyfills to work under C++11, it should not be a case with a spdlog project that just links with it

[jabartek]

I'm not sure how exactly #1981 passed the GCC_VERSION=4.8 BUILD_TYPE=Release CPP=11 check - maybe this should be looked into as well. Sadly, I am not a CI person, so cannot really help with that

Okay, it makes perfect sense, that sinks aren't built during CI checks, especially those linking with outside libs. But still - C++11 compilance should stand

[gabime]

Thanks @jabartek