Update to Lager 2.0 and rebar build with R16

This is a modernization commit more than anything.
It brings the versions of the dependencies up
to a more recent state.
Bring back checks for R15

At aircloak we are no longer using R15,
but since this is open source, we should
keep backward compatibilty with R15 a while
longer
Merge pull request #5 from Aircloak/sebastian/lager-2

Sebastian/lager 2