Fix redsocks_evbuffer_readline with libevent 2.1

[apoikos]

_EVENT_NUMERIC_VERSION was renamed to EVENT__NUMERIC_VERSION in libevent 2.1. As a result, redsocks_evbuffer_readline would end up using evbuffer_readline(buf), which causes client connections to hang indefinitely.

Switch the check to using LIBEVENT_VERSION_NUMBER instead. LIBEVENT_VERSION_NUMBER has been around since libevent 2.0.3 and redsocks is already using it in other parts of the code.

Fixes #107, #122.

[houtmanj]

This change fixed my issue on archlinux with redsocks and libevent 2.1. Works like a charm now

[agherzan]

Any chance we can get this in soon?

[matlec]

I spent some time debugging this problem until I fixed the problem in my local source tree and discovered this issue. Please merge the pull request proposed by @apoikos, otherwise Redsocks is not usable with http-connect.