Update "upcoming release" of tilt in README

Version 2.0 of Tilt is apparently out, so the README shouldn't state that v2.0 is still upcoming.
Target note in README to users of Tilt v1.4

Fix note at the top of the README to make it more useful to users of Tilt v1.4.
Merge pull request #227 from Ajedi32/patch-1

Update "upcoming release" of tilt in README