Revert Dependency Check goal from 'purge' to 'check' once NVD API stops returning 503 'Service Unavailable' errors

[kwwall]

For the branch 'develop', I intentionally left the pom.xml in a state where the default goal for the Dependency Check plugin is set to 'purge' rather than 'check'. That's just to prevent a lot of frustration for those who try running 'mvn site' or 'mvn install', etc. or anything else that triggers the Dependency Check plugin. For the last 3 or 4 days, it has been returning errors and failing (although usually only after hanging for 10 minutes or so) because eventually the NIST NVD API returns a 503 HTTP status code.

This is just a reminder to fix the pom once NIST NVD seems to have addressed this issue.