Bump to latest nexus version

Security advisory: https://support.sonatype.com/entries/84705937-CVE-2014-9389-Nexus-Security-Advisory-Directory-Traversal

Upgrade notes: https://support.sonatype.com/entries/21701998-Sonatype-Nexus-Upgrade-and-Compatibility-Notes

I've tested the upgrade with a basic deployment on CentOS 6.5. (Note the official :external_version url actually has the -01 in its path for this release, so I'm using that.)