test: Fix npm test failures on Travis CI

* Remove node 0.6
* Quote values in .travis.yaml because in javascript/JSON/YAML
  0.1 === 0.10.
* Fix dependencies at a slightly less wildcard-y number. Fixed
  them at tilde ranges of the current latest release.
* Remove redundant engine setting. This could be set to >= 0.8.0
  since the dependencies have a highest-common denominator effect.
  But npm-install takes care of that for us.