Check whether PLT file is outdated or not

Erlang's dialyzer creates a PLT file for static analysis. Creating
such a file takes a long time, hence it's preserved between builds
on the build bots.

It can happen that the PLT file is outdated and needs to be
regenerated. This commits adds the automated regeneration.

Change-Id: I1194c8db44a662821e8f55d435ee2e34d4c61707
Reviewed-on: http://review.couchbase.org/41814
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>