[build] Enable staged version of Tycho 0.19.0 for testing

Change-Id: If00175b8c81a1b60a1378d394efcbdecddc1adb1
[build] Revert back to Tycho 0.18.1 until 0.19.0 is officially released

This reverts commit 488dc4ce0a634bc95a70e0d619def709d51aa76b.