Fix #57 - incompatible with r10k gem > 3.8

  * pin r10k to >= 2.6.x

  * Without the pinned version, The dependency solver will
    will resolve to r10k 1.x which fails hard when used.

    At a minimum 2.x seems to be required

ra10ke.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "rake"
   spec.add_dependency 'puppetfile-resolver'
   spec.add_dependency "puppet_forge"
-  spec.add_dependency "r10k"
+  spec.add_dependency "r10k", '>= 2.6.5'
   spec.add_dependency "git"
   spec.add_dependency "solve"
   spec.add_dependency 'semverse', '>= 2.0'