Fix NoMethodError when jshint.js array has nil elm

Sometimes the array from `jshint.js` will contain `nil` elements.
For example, when there are "too many errors", `jshint.js` will
return an array whose last element is `nil`. The array returned by
`run_lint` omits any such `nil` elements.
Merge pull request #8 from jaredbeck/compact_the_error_array

Fix NoMethodError when jshint.js array has nil elm