fix download link to binary file

updated the link and it seems to work now :)

Darwin 2 kaizhu@minikai:~/src/slimerjs $ node install.js
Downloading http://download.slimerjs.org/releases/0.9.1/slimerjs-0.9.1-mac.tar.bz2
Saving to /var/folders/h4/pg57cdtx2yq3tn0mww9tkjpc0000gp/T/slimerjs/slimerjs-0.9.1-mac.tar.bz2
Receiving...
Received 782K...
Received 1564K...
Received 2346K...
Received 3129K...
Received 3911K..

Received 39395K...
Received 39399K total.
Extracting tar contents (via spawned process)
Copying extracted folder /var/folders/h4/pg57cdtx2yq3tn0mww9tkjpc0000gp/T/slimerjs/slimerjs-0.9.1-mac.tar.bz2-extract-1401971251343/slimerjs-0.9.1 -> /Users/kaizhu/src/slimerjs/lib/slimer
Writing location.js file
Done. Slimerjs binary available at /Users/kaizhu/src/slimerjs/lib/slimer/slimerjs
Darwin 2 kaizhu@minikai:~/src/slimerjs $