Fix jsperf benchmarks

The jsperf benchmarks linked in the readme no longer work, since raw.github.com serves as text/plain and disables content type sniffing \<script\> inclusions to raw.github.com are denied by modern web browsers. 

Someone edited the benchmarks to fix this problem using rawgithub.com in revisions http://jsperf.com/simplex-noise/4 and http://jsperf.com/simplex-noise-comparison/3; this PR updates the readme to link to the updated URLs.