Restore rack dependency because http input needs it

Fixes elastic#3634

Bump version to 1.4.4

Fixes elastic#3624

Update lock file to use 1.5.3.snapshot2

Update lock file for 1.5.3.snapshot1

Fixes elastic#3597

take 2, use 1.5.2.2 logstash-core

Fixes elastic#3539

Use the new 1.5.2.snapshot2

Fixes elastic#3531

Bump version to 1.5.2.snapshot1

Fixes elastic#3522

Update lock file for 1.5.1 release

Fixes elastic#3441

Use latest 1.5.1.snapshot1 logstash-core

Fixes elastic#3421

Make some small changes to make docs build.

Also updated the Gemfile locks to update some versions of things.