gem: stick with Bundler 1.x until bundle-audit has a new Bundler 2 compatible release

.travis.yml

@@ -5,3 +5,7 @@ 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'
+  s.add_development_dependency 'bundler', '< 2'
   s.add_development_dependency 'rake'
   s.add_development_dependency 'rspec'
   s.add_development_dependency 'rack-test'