Fix #117 add dependency of tilt gem

I found that it is because of guard/jasmine/coverage requires tilt,
but gemspec not includes dependency of tilt.

please test it on new directory only includes Gemfile

```ruby
gem 'guard-jasmine', :git => 'git://github.com/masarakki/guard-jasmine', :branch => 'add_dependency_of_tilt'
```

and run

```
 bundle exec guard init
```