Updating Tomcat 6 version to fix broken bootstrap process

The bootstrap process is currently failing as a result of 404s being thrown while trying to pull down apache-tomcat-6.0.37, which is no longer the latest stable release.  I've updated the script to reference the latest stable release (6.0.39) and pulled the version out into a variable that can be more easily updated in the future.

The alternative would be to point the script to the archive location for apache-tomcat-6.0.37 and remain on the older version, but I've tested the bootstrap process with apache-tomcat-6.0.39 and didn't run into any problems.