Make age option an integer; use age in URL when querying API

* using to_i on the option `age` fixes the following error when specifying --age/-A:
```
CheckAggregate CRITICAL: Check failed to run: String can't be coerced into Fixnum, 
["/etc/sensu/plugins/check-aggregate.rb:116:in `-'", "/etc/sensu/plugins/check-aggregate.rb:116:in 
`get_aggregate'", "/etc/sensu/plugins/check-aggregate.rb:167:in `run'", "/opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-plugin-0.1.7/lib/sensu-plugin/cli.rb:56:in `block in <class:CLI>'"]
```
* adding `?age=` to the api_request in `get_aggregate`