use single quotes in spec to make Travis CI happy

Author: Matt Zukowski

spec/grape/validations/params_scope_spec.rb

@@ -126,7 +126,7 @@ class AnotherCustomType
           def self.parse(value)
             case value
             when 'invalid with message'
-              Grape::Validations::Types::InvalidValue.new "is not correct"
+              Grape::Validations::Types::InvalidValue.new 'is not correct'
             when 'invalid without message'
               Grape::Validations::Types::InvalidValue.new
             else