fixed that the gem links against different openssl library than eventmachine and ruby

eventmachine >=  0.12.6 prefers pkg_config('openssl') over dir_config('ssl') to determine which openssl library to compile and link against.

evma_httpserver needs to follow this strategy, otherwise there will be two different versions of libssl active in a running process, which can lead to all sorts of undefined behavior, including segfaults.

Could you please merge and release a new version?