Corrected the setup instructions in the README

The environment variables need to be set before running node app.js to
prevent an OAUTH error message.

Error message before changing the order of the steps:

pullup/node_modules/passport-github/node_modules/passport-oauth/lib/passport-oauth/strategies/oauth2.js:66
  if (!options.clientID) throw new Error('OAuth2Strategy requires a clientID o
                               ^
Error: OAuth2Strategy requires a clientID option