Fix TypeError: Cannot read property 'bold' of undefined

Currently `yo bones` yield a 
```
TypeError: Cannot read property 'bold' of undefined
    at Object.<anonymous> (/usr/local/share/npm/lib/node_modules/generator-bones/node_modules/yeoman-generator/lib/util/common.js:5:56)
```

This fixes the issue
Merge pull request #3 from p-j/patch-1

Fix TypeError: Cannot read property 'bold' of undefined