Update bundler dependency to allow any version >= 1.15

Signed-off-by: Renato Westphal <[email protected]>

Author: rwestphal

netgen.gemspec

@@ -31,7 +31,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.3.0'
   spec.add_dependency 'ffi'
-  spec.add_development_dependency 'bundler', '~> 1.15'
+  spec.add_development_dependency 'bundler', '>= 1.15'
   spec.add_development_dependency 'rake', '~> 10.0'
   spec.add_development_dependency 'rspec', '~> 3.0'
 end