Mitigate `npm test` permission error

`sh node_modules/.bin/vows` should work, as `vows` has a good shebang line. However, on Mac OS X, somehow `vows` is still being interpreted as a shell script:

    node_modules/.bin/vows: line 4: syntax error near unexpected token `('
    node_modules/.bin/vows: line 4: `var path   = require('path'),'
    npm ERR! Test failed.  See above for more details.
    npm ERR! not ok code 0

Fortunately, this is easily fixed by manually calling `vows` with `node` rather than `sh`.