workaround for cURL related crashes on some systems

This is a workaround for the issue described here https://github.com/typhoeus/typhoeus/issues/260. 

Not sure if this is the best way to fix this, since there's no current way to pass options directly to the adapter. I think it should be safe enough to set this flag all the time, but not 100% sure. 

Some more details on that particular flag: http://curl.haxx.se/libcurl/c/CURLOPT_NOSIGNAL.html