Timeout handling between Async and Non Async calls is not consistent

When attempting to set the default timeout interval for jasmine-node, I currently need to set both of the following values.  

jasmine.getEnv().defaultTimeoutInterval = 15000;
jasmine.DEFAULT_TIMEOUT_INTERVAL = 15000;

This is caused by the fact that the async-callback.js (line 41) code refers the 'DEFAULT_TIMEOUT_INTERVAL', while the rest of the jasmine code refers to the getEnv().defaultTimeoutInterval.  

This change uses getEnv().defaultTimeoutInterval so that it is only required to set this value one time for a group of tests.