d/watch: point to github as they removed tests from the gem

See upstream https://github.com/rtomayko/tilt/pull/339
8 jobs for master in 6 minutes and 48 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #374096
build

00:03:18

 
  Test
passed #374101
autopkgtest

00:03:22

passed #374102
blhc

00:01:15

passed #374100
lintian

00:01:29

passed #374103
piuparts

00:03:10

passed #374099
reprotest

00:03:09

passed #374098
test-build-all

00:03:29

passed #374097
test-build-any

00:02:11