Fix webmock errors in specs for ruby 2.1 and 2.2

New versions of webmock released within the last month have adopted the
ruby 2.4 method invocation syntax of &. but the tested ruby versions do
not support it.

This commit resolves the problem by conditionally bundling either the
older version of webmock or newer versions based on the current ruby
version.

Gemfile

@@ -13,7 +13,8 @@ group :test do
   gem "rspec", ">= 3.2"
   gem "rspec-mocks", ">= 3"
   gem "simplecov", ">= 0.13"
-  gem "webmock"
+  gem "webmock", '~> 3.14.0' if Gem::Version.new(RUBY_VERSION) < Gem::Version.new("2.4.0")
+  gem "webmock", '>= 3.14' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("2.4.0")
 end
 
 gemspec