Updates latest version of underscore.js (1.6.0 to 1.7.0)

Updated underscore.js to its latest version (1.7.0)
```
~> bundle exec rake
Finished in 2.237419s, 9.8328 runs/s, 111.7359 assertions/s.
22 runs, 250 assertions, 0 failures, 0 errors, 0 skips
```