pangyp required at runtime for post-install from build.js

[hmalphettes]

➜  tmp  npm i node-sass
|
> node-sass@2.0.1 install /Users/hugues/proj/tmp/node_modules/node-sass
> node scripts/install.js

Can not download file from https://raw.githubusercontent.com/sass/node-sass-binaries/v2.0.1/darwin-x64-iojs-1.4/binding.node

> node-sass@2.0.1 postinstall /Users/hugues/proj/tmp/node_modules/node-sass
> node scripts/build.js

module.js:322
    throw err;
          ^
Error: Cannot find module '/Users/hugues/proj/tmp/node_modules/node-sass/node_modules/pangyp/bin/node-gyp'
    at Function.Module._resolveFilename (module.js:320:15)
    at Function.Module._load (module.js:262:25)
    at Function.Module.runMain (module.js:485:10)
    at startup (node.js:112:16)
    at node.js:863:3
Build failed

pangyp is assumed to be installed as a local module by script/build.js.
But it is only installed for development.

This PR fixes that issue.

[hmalphettes]

Well, when I am using the patched module with iojs-v1.4.1 from my main application pangyp behaves strangely.
I ended-up reverting back to using the standard-node-gyp and all is well:
https://github.com/hmalphettes/node-sass/commit/5a113952bf517ebb96f50204431b8f2d8844500d