travis: use iojs 2.5
travis: use iojs 2.5

To work around https://github.com/nodejs/node/issues/2581

iarna/node-tap,myndzi/node-tap,jinivo/marhert,isaacs/node-tap,myndzi/node-tap,othiym23/node-tap,jkrems/node-tap,strongloop-forks/node-tap,jkrems/node-tap,othiym23/node-tap,tapjs/node-tap,strongloop-forks/node-tap,isaacs/node-tap,tapjs/node-tap,evanlucas/node-tap,evanlucas/node-tap,iarna/node-tap