Fix Rack dependency issue for Ruby < 2.2.2
Fix Rack dependency issue for Ruby < 2.2.2

bdshroyer/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,joemoore/LicenseFinder,sschuberth/LicenseFinder,bdshroyer/LicenseFinder,bspeck/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,bdshroyer/LicenseFinder,pivotal/LicenseFinder,bspeck/LicenseFinder,bspeck/LicenseFinder,tinfoil/LicenseFinder,tinfoil/LicenseFinder,joemoore/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,joemoore/LicenseFinder,sschuberth/LicenseFinder,joemoore/LicenseFinder,pivotal/LicenseFinder,pivotal/LicenseFinder,bspeck/LicenseFinder,tinfoil/LicenseFinder,bdshroyer/LicenseFinder,joemoore/LicenseFinder,tinfoil/LicenseFinder,bdshroyer/LicenseFinder,pivotal/LicenseFinder,bspeck/LicenseFinder,tinfoil/LicenseFinder