Bug fix #4.

#4 has a bug.
`present?` method is custom method of RoR.

```
[2014-04-28T08:46:10+00:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
[2014-04-28T08:46:10+00:00] DEBUG: NoMethodError: undefined method `present?' for "":String
/home/vagrant/chef-solo/cookbooks-2/awscli/recipes/default.rb:14:in `block in from_file'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.10.4/lib/chef/dsl/recipe.rb:123:in `instance_eval'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.10.4/lib/chef/dsl/recipe.rb:123:in `build_resource'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.10.4/lib/chef/dsl/recipe.rb:86:in `declare_resource'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.10.4/lib/chef/dsl/recipe.rb:42:in `method_missing'
/home/vagrant/chef-solo/cookbooks-2/awscli/recipes/default.rb:12:in `from_file'
```