Skip if getrusage is not supported
Skip if getrusage is not supported

1243255c3a36433041012b6107a5ac48658a0895 broke ci.rvm.jp tests like
http://ci.rvm.jp/results/trunk_clang_39@silicon-docker/2445098.

253232c028a5565dbeecc05fab5e81b35ab58bcc works only if getrusage is supported.

nobu/rubyspec,ruby/spec,eregon/rubyspec,ruby/spec,nobu/rubyspec,eregon/rubyspec,ruby/spec,eregon/rubyspec,nobu/rubyspec