Support for ruby-head (version 3)

[galois17]

There's an error with em-http-request at version 0.2.11 with ruby 3.0.
make "DESTDIR=" compiling em_buffer.c em_buffer.c:8:10: fatal error: 'rubyio.h' file not found #include "rubyio.h" ^~~~~~~~~~ 1 error generated. make: *** [em_buffer.o] Error 1
The latest version of em-http-request doesn't have this issue. The fix for ruby 3.0 would be to use em-http-request ~> 1.1.7, but this requires some code change.

[pboling]

@galois17 please test out latest master. I've made all the changes for latest em-http-request that I could find!

[pboling]

Ref: Closed by #176