Update tarball URL to avoid HTTP 404

Looks like sourceforge.net changed their URL mapping for downloading a file, current URL produces a 404:

$  npm install optipng-bin
...
> optipng-bin@2.0.3 postinstall /path/to/node_modules/optipng-bin
> node lib/install.js

  ⚠ /path/to/optipng-bin/vendor/optipng 0.7.4 does not satisfy the version requirement of >=0.7.5
  ⚠ optipng pre-build test failed
  ℹ compiling from source
  ✖ Error: Couldn't connect to http://downloads.sourceforge.net/project/optipng/OptiPNG/optipng-0.7.5/optipng-0.7.5.tar.gz (404)
    at Request.<anonymous> (/path/to/node_modules/optipng-bin/node_modules/bin-build/node_modules/download/index.js:147:16)
    at Request.EventEmitter.emit (events.js:95:17)
    at Request.onRequestResponse (/path/to/node_modules/optipng-bin/node_modules/bin-build/node_modules/download/node_modules/request/request.js:1255:10)
    at ClientRequest.EventEmitter.emit (events.js:95:17)
    at HTTPParser.parserOnIncomingClient [as onIncoming] (http.js:1688:21)
    at HTTPParser.parserOnHeadersComplete [as onHeadersComplete] (http.js:121:23)
    at Socket.socketOnData [as ondata] (http.js:1583:20)
    at TCP.onread (net.js:525:27)

Fix consists of adding /download to the URL.