Update version to 2.0.0 as '2.0' breaks Composer extensions "require"

I never thought this is a change I'd get to commit! :-)

Update version to 2.0.0 as '2.0' breaks Composer extensions that require '2.0.0', e.g.

"require" : {
    "bolt/bolt" : ">=2.0.0,<3.0.0"
},