updating node modules

- `npm run outdated` &mdash; this will check for outdated dependencies and devDependencies and display newer version numbers in the Travis log (but not fail anything ever).
- `grunt validate-package --force` &mdash; this will check for potentially vulnerable modules in the package.json file using the nodesecurity.io database. The `--force` flag means that the exit code will always be 0 and not fail the build. If we start using shrinkwrap (see #15) then I'll switch to `grunt validate-package` instead since it is more thorough and performant.
- Made a tweak to `grunt lint` so it also works without a specified target (now defaults to `lint:prebuild`, but previously it always expected to be called explicitly with `grunt lint:prebuild` or `lint:postbuild` due to some previous htmllint task changes I [apparently hastily] made.

r= @nchapman

Fixes #264