Fixed undefined var when generating without Bootstrap

Bombs out when it attempts to evaluate `btnDanger` for line 42

```
undefined:38
((__t = ( btnDanger )) == null ? '' : __t) +
          ^
ReferenceError: btnDanger is not defined
    at eval (eval at template (/usr/local/lib/node_modules/generator-angularfire/node_modules/yeoman-generator/node_modules/lodash/dist/lodash.js:6305:22), <anonymous>:38:11)
    at Generator.underscore [as _engine] (/usr/local/lib/node_modules/generator-angularfire/node_modules/yeoman-generator/lib/util/engines.js:32:30)
    at Generator.engine (/usr/local/lib/node_modules/generator-angularfire/node_modules/yeoman-generator/lib/actions/actions.js:303:10)
    at Generator.template (/usr/local/lib/node_modules/generator-angularfire/node_modules/yeoman-generator/lib/actions/actions.js:281:15)
    at Generator._htmlTpl (/usr/local/lib/node_modules/generator-angularfire/app/index.js:490:8)
    at Generator.copyAngularFireFiles (/usr/local/lib/node_modules/generator-angularfire/app/index.js:418:10)
    at /usr/local/lib/node_modules/generator-angularfire/node_modules/yeoman-generator/lib/base.js:341:43
    at /usr/local/lib/node_modules/generator-angularfire/node_mo%
```

Was going to write a test for this, but I don't see a test task on Grunt. How can I run them?