Update maven-javadoc-plugin to 2.7 (Maven build broken)

The existing version (2.6.1) of maven-javadoc-plugin used in the build can cause a top-level build to fail on Maven 2.2.1:

[INFO] Unable to find resource 'org.easymock:easymock:jar:3.1-SNAPSHOT' in repository snapshots (http://snapshots)
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Failed to resolve artifact.

Missing:
----------
1) org.easymock:easymock:jar:3.1-SNAPSHOT

I believe that this is a known problem with this plugin, which can be fixed by moving up to version 2.7.

POM patch attached.