.travis.yml: drop node 0.11 from list for now.

Getting a strange error from python about node-gyp v0.12.1 when
installing node-LDAP with node v0.11.9 in the Travis-CI VM.

Log: https://travis-ci.org/tcort/nodebook/jobs/14873292

Works ok on node 0.8 and 0.10, so I'll stick with that for now
as I have a tight deadline. This issue should be revisited down
the road as the nodejs version numbers get higher.