Addresses errors reported in gdg-x/generator-boomerang#5

Fix was to rev the yeoman-generator version,. Yeoman changed some of the greeting text between versions causing the error.

To make sure the old version is gone, run (use sudo if applicable)
npm cache clean
npm uninstall -g yeoman-generator 
npm install -g yeoman-generator
npm uninstall -g generator-boomerang

After that rerun, 
npm install -g generator-boomerang

to get updated dependencies and everything should work.