Basic Gulp v4 compatibility

[Jimbly]

Key issues with Gulp v4 are: gulp.task never takes a deps argument (deps are now done via gulp.task('foo', gulp.series('dep', fn)), and all task dependencies must be defined, even if not called in a context (e.g. in the worker process, can't rename a task without also leaving something under the old name (which will never be called) for gulp's validation code to find).

This change should keep this module compatible with both Gulp v3 and v4.

After fixing the Gulp v4 issues, adding the one line of require('gulp-ll').tasks(['eslint']) nearly cut my build times in half, thanks!