removed limitation on version of yajl-ruby ~> 0.7.6

<pre><code>dyld: lazy symbol binding failed: Symbol not found: _yajl_set_static_value
  Referenced from: /opt/boxen/rbenv/versions/1.9.3-p385-perf/lib/ruby/gems/1.9.1/gems/yajl-ruby-0.7.9/ext/yajl/yajl.bundle
  Expected in: flat namespace

dyld: Symbol not found: _yajl_set_static_value
  Referenced from: /opt/boxen/rbenv/versions/1.9.3-p385-perf/lib/ruby/gems/1.9.1/gems/yajl-ruby-0.7.9/ext/yajl/yajl.bundle
  Expected in: flat namespace</code></pre>

^^ get the above errors in older versions of yajl-ruby. Works fine with yajl-ruby-1.1.0. If you're cool with it, remove the restriction from the gemspec on yajl-ruby ~> 0.7.6 and allow >= instead.