Separate Travis Builds

Travis supports running a matrix of separate builds. This (a)
paralellizes the build process and (b) makes it easier to see when
the specs are failing on one particular environment (set of
dependencies).

This commit

 1. introduces a new "test" task that runs `script/run` against the
    current environment (with no prerequisites) and uses that in Travis
 2. eliminates Travis builds on Node 0.11. That build was mostly
    redundant since this project isn't terribly sensitive to changes
    in Node.js.
 3. moves all dependency installation on Travis into its install step
 4. adds a long series of test configurations to the `travis.yml`
    `env` matrix

References:

 * http://docs.travis-ci.com/user/build-configuration/
 * http://docs.travis-ci.com/user/caching/
 * http://docs.travis-ci.com/user/languages/javascript-with-nodejs/