Revert "depend on nathan7/next-tick for now"

This reverts commit 845154ad01ca941554f9442c40f2c72256f28fd9.
timoxley/next-tick#5 got merged so there's no need to depend on the
fork.
Merge pull request #8 from nathan7/timoxley-next-tick

Revert "depend on nathan7/next-tick for now"