Replace deprecated getdtablesize with sysconf

[Kjarrigan]

#166 (comment)

@tevio wrote:
As per the linux foundation deprecation note here:-
http://refspecs.linux-foundation.org/LSB_4.0.0/LSB-Core-generic/LSB-Core-generic/baselib-getdtablesize.html

This change keeps the gem up to date as this function is removed from various platforms and allows it to run on aarch32/64.

The MR will be reverted soon as the side effects were too much for a lot of ethon/typhoeus users.

So creating this issue so one might find a different solution to the problem.

[thatsmydoing]

Since ruby 2.2.0, there's Etc.sysconf

The ffi gem already needs ruby 2.3.0 so it shouldn't be a problem.