WIP: Update to Nx 2.6 and enforcing java6 level

But currently, all the IT craps out as it tries to
load up wrong maven properties file (nexus-testsuite-support
vs nexuspro-testsuite-support)...
Fixed ITs and typos.
Merge pull request #11 from sonatype/update-nx-26

NEXUS-5731: Enforce no Java7 refs creep in plugins + update to Nx 2.6