Adding support of older systems, with GCC 4.9 and cmake 3.6.2 (Debian 8)

[braindigitalis]

sleepy-discord wouldn't build on my older Debian system. With some time to do so, and not having the ability to simply upgrade the server, I've built a small patch-set that fixes the issues. Please merge in at your leisure if you find this useful.

The three commits are as follows:

1. One-line change to sleepy_discord/CMakeLists.txt to allow older cmake 3.6 to find and use C++14
2. Small change to sleepy_discord/client.cpp in rate limiting code, changes single use of std::get_time() to strptime(), which does the same thing but is the C style way, better backwards compatibility.
3. Some small and simple changes to json.c to stop it complaining about declarations of variables inside for loop keywords.

[braindigitalis]

After updating to newer cmake and also moving to develop branch, only the get_time() part of this merge is required there. This cuts down the patch significantly.

[yourWaifu]

the json.c stuff has been removed and right now I'm merging develop branch, so there's going to be merge conflicts.