handle lack of `execSync` gracefully

Currently `scripts/lint.js` depends on an external module.
However, this module doesn't build on all systems so isn't bundled within
package.json. This commit adds a check to ensure that `execSync` exists, so
there aren't errors when someone tries to commit something on a machine without
that module.

Ideally, we would migrate away from `execSync()` and use `fs.sync()` instead.
Merge branch 'master' of github.com:zuzak/cinequeue
fix malformer wercker.yml