Fix merge with an already existing Change-Id

The merge failed when another review had the same Change-Id (if the
review was abandoned or on another branch for instance).

Change-Id: If324e309744d6386267f4a44b948291edf3e6ebb
Fix Travis build

The plugin is only compatible with the Gerrit branch 2.10 for the
moment.

Change-Id: I3ecf2c4f3c321df3d8b7e1e0bf7888c77c14cf44
Merge pull request #2 from debanne/master

Fix merge with an already existing Change-Id