Fix for EM/em-http-request close_connection

If the em-http-request gem is installed, then camper_van will use it as
a connection.  It does not have a .close_connection method, but rather
a .close

https://github.com/igrigorik/em-http-request/blob/master/lib/em-http/htt
p_connection.rb#L172-201
Merge pull request #44 from tleish/master

Fix for EM/em-http-request close_connection