Build node-sqlite3 for node-webkit on Travis CI

Travis CI configuration changes proposed in #251.

* [x] fork node-sqlite3

* [x] try tweaking the .travis.yml to include a matrix for different node-webkit versions

* [x] download the right stuff

* [x] build

* [ ] run `node-pre-gyp package testpackage`

Note: there's currently no API to determine the most recent version of node-webkit over the Web. And thus the desired node-webkit's version is given after Node's version in the matrix.