Upgrade WebMock

To include Ruby 2.4 support:
bblimke/webmock#683

Author: myersjustinc

bls_api.gemspec

@@ -28,5 +28,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "climate_control", "~> 0.0.3"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec"
-  spec.add_development_dependency "webmock", "~> 1.21"
+  spec.add_development_dependency "webmock", "~> 2.3"
 end