drop version lock on bundler

we want to support 1.x and 2.x, chances are 3.x will work fine as well
since they can properly handle lock files properly backwards compatible.
therefore the version lock doesn't really help much and so we don't have
to revisit this for 3.x explicitly we just don't version lock bundler
at all. should 3.x be in fact super not compatible we can always backpaddle