Adds sprockets dependency to avoid having to run bundle install.

Hey @nicklink483 

This PR is a fix for issue #7.

Upon running `gem install`, sprockets-3.0.0.beta.6 would get installed, the latest version. However, when running `bundle show` you would get this: 
```
➜  example1  bundle show
Could not find gem 'sprockets (< 4.0.0, >= 2.2.3) ruby', which is required by gem 'opal (= 0.7.0beta3) ruby', in any of the sources.
```
and when running `bundle install`: 
```
Installing sprockets 2.12.3
```

So apparently, a gem is having a dependency on a lower version number of sprockets. By adding this requirement to gemspec: 
```
➜  dare-example3  gem install ~/Projects/dare/dare-0.2.0.gem --no-ri --no-rdoc
Fetching: rake-10.4.2.gem (100%)
Successfully installed rake-10.4.2
Fetching: bundler-1.7.9.gem (100%)
Successfully installed bundler-1.7.9
Fetching: sprockets-2.12.3.gem (100%)
Successfully installed sprockets-2.12.3
Fetching: sourcemap-0.1.1.gem (100%)
Successfully installed sourcemap-0.1.1
Fetching: opal-0.7.0.beta3.gem (100%)
Successfully installed opal-0.7.0.beta3
Fetching: opal-jquery-0.3.0.beta1.gem (100%)
Successfully installed opal-jquery-0.3.0.beta1
Fetching: thor-0.19.1.gem (100%)
Successfully installed thor-0.19.1
Successfully installed dare-0.2.0
8 gems installed
➜  dare-example3  dare new game3
      create  game3/Gemfile
      create  game3/Rakefile
      create  game3/game3.rb
      create  game3/game3.html
➜  dare-example3  cd game3
➜  game3  rake build
➜  game3  ls -la
total 936
   272 Dec 23 14:19 .
   136 Dec 23 14:19 ..
   105 Dec 23 14:19 Gemfile
   760 Dec 23 14:19 Gemfile.lock
   502 Dec 23 14:19 Rakefile
   428 Dec 23 14:19 game3.html
458236 Dec 23 14:19 game3.js
   248 Dec 23 14:19 game3.rb
➜  game3  bundle
Using rake 10.4.2
Using hike 1.2.3
Using bundler 1.7.9
Using sourcemap 0.1.1
Using multi_json 1.10.1
Using rack 1.4.5
Using tilt 1.4.1
Using thor 0.19.1
Using sprockets 2.12.3
Using opal 0.7.0.beta3
Using opal-jquery 0.3.0.beta1
Using dare 0.2.0
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
```