switch gemspec to optimistic constraints

- Prevent dependencies on outdated gems.
- Cucumber 3.1 is now a minimum version, not a fixed dependency.

guard-cucumber.gemspec

@@ -17,11 +17,11 @@ Gem::Specification.new do |s|
 
   s.required_rubygems_version = ">= 1.3.6"
 
-  s.add_dependency "cucumber",    "~> 3.1"
-  s.add_dependency "nenv", "~> 0.1"
+  s.add_dependency "cucumber", ">= 3.1"
+  s.add_dependency "nenv", ">= 0.1"
 
-  s.add_development_dependency "guard-compat", "~> 1.0"
-  s.add_development_dependency "bundler", "~> 1.6"
+  s.add_development_dependency "guard-compat", ">= 1.0"
+  s.add_development_dependency "bundler", ">= 1.6"
 
   s.files = `git ls-files -z`.split("\x0").select do |f|
     /^lib\// =~ f