As far as I can tell, `curly, multi-line, consistent` does everything I
want; there's no reason to keep around a validate-conditionals rule.
Which is probably good, since eslint is deprecating the API for this,
anyway. The nice thing about not relying on deprecated APIs is that now
you can lint PS with `eslint` rather than needing to memorize
command-line switches.
JSHint's ES6 support is shaky, and its development has stalled as of late.
Since ESLint can do by itself both JSHint and JSCS' jobs, this commit replaces them.
Gulp and its related dependencies are also hereby removed.
