Removed chef from Gemfile

I have removed chef from the global Gemfile as it is not used here anymore
and it just conflicts with locally installed chef versions so that Guard
is not usable on the workstation.
Removed knife tools and foodcritic

As foodcritic is not used here we can simply remove the
overhead, the same for the knife tools.
Set less strict gem version constraints

We don't need to set too strict version constraints here,
newer versions of the development gems are totally ok.
Set a minimal version for pry-debugger
Replaced quotes in a unified manner
Merge pull request #2038 from tboerger/feature/gemfile-cleanup

Removed chef from Gemfile
fix ruby syntax