gem: do not restrict Bundler version anymore since bundle-audit 0.6.1 is Bundler 2 compatible now

.travis.yml

@@ -6,7 +6,3 @@ rvm:
   - 2.5
   - 2.4
   - 2.3
-
-before_install:
-  - gem uninstall -x -i /home/travis/.rvm/gems/ruby-2.3.7@global bundler || true
-  - gem install bundler -v '< 2'

dyndnsd.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'rack-tracer', '~> 0.8.0'
   s.add_runtime_dependency 'jaeger-client', '~> 0.10.0'
 
-  s.add_development_dependency 'bundler', '< 2'
+  s.add_development_dependency 'bundler'
   s.add_development_dependency 'rake'
   s.add_development_dependency 'rspec'
   s.add_development_dependency 'rack-test'