Move CACHE_STORE_DIR assignment after setting NPM_VERSION

Add version to the sample package.json shown in README.

Without this `myapp` throws the following when deployed to Heroku:

    -----> Installing dependencies with npm
           npm ERR! Couldn't read dependencies.

           npm ERR! Error: 'version' field missing
           npm ERR! 'version' Must be X.Y.Z, with an optional trailing tag.
           npm ERR! See the section on 'version' in `npm help json`
           npm ERR!     at /tmp/node-npm-a99U/lib/utils/read-json.js:336:13
           npm ERR!     at /tmp/node-npm-a99U/lib/utils/read-json.js:134:32
           npm ERR!     at P (/tmp/node-npm-a99U/lib/utils/read-json.js:110:40)
           npm ERR!     at [object Object].<anonymous> (fs.js:115:5)
           npm ERR!     at [object Object].emit (events.js:64:17)
           npm ERR!     at afterRead (fs.js:1111:12)
           npm ERR!     at Object.wrapper [as oncomplete] (fs.js:254:17)
           npm ERR! You may report this log at:
           npm ERR!     <http://github.com/isaacs/npm/issues>
           npm ERR! or email it to:
           npm ERR!     <npm-@googlegroups.com>
           npm ERR!
           npm ERR! System Linux 2.6.32-342-ec2
           npm ERR! command "/tmp/node-node-z9p0/bin/node" "/tmp/node-npm-a99U/cli.js" "install"
           npm ERR! cwd /tmp/build_1qpkl3ftzyifk
           npm ERR! node -v v0.6.10
           npm ERR! npm -v 1.1.1
           npm ERR! message 'version' field missing
           npm ERR! message 'version' Must be X.Y.Z, with an optional trailing tag.
           npm ERR! message See the section on 'version' in `npm help json`
           npm ERR!
           npm ERR! Additional logging details can be found in:
           npm ERR!     /tmp/build_1qpkl3ftzyifk/npm-debug.log
           npm not ok
     !     Failed to install dependencies with npm
     !     Heroku push rejected, failed to compile Node.js app

README.md had typo for buildpacks documentation link