constrain cucumber (> 3.1, < 4.0)

Guard::Cucumber::NotificationFormatter breaks on Cucumber 4.0.0, and
possibly others in the 4.x series.

Author: todd-a-jacobs

guard-cucumber.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |s|
 
   s.required_rubygems_version = ">= 1.3.6"
 
-  s.add_dependency "cucumber", ">= 3.1"
+  s.add_dependency "cucumber", ">= 3.1", "< 4.0"
   s.add_dependency "guard-compat", ">= 1.0"
   s.add_dependency "nenv", ">= 0.1"