Relax development dependencies

Don't pin development dependencies to specific (old) versions.

It could make sense to change several dependencies from `~>` to `>=`.
And _maybe_ keep `rspec` and `webmock` with `~>`.  But, at least for gem
development, I've found that's usually much more hassle than it's worth.

Author: nevans

vault.gemspec

@@ -26,11 +26,11 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "connection_pool",     "~> 2.4"
   spec.add_runtime_dependency "net-http-persistent", "~> 4.0", ">= 4.0.2"
 
-  spec.add_development_dependency "bundler", "~> 2"
-  spec.add_development_dependency "pry",     "~> 0.13.1"
-  spec.add_development_dependency "rake",    "~> 12.0"
-  spec.add_development_dependency "rspec",   "~> 3.5"
-  spec.add_development_dependency "yard",    "~> 0.9.24"
-  spec.add_development_dependency "webmock", "~> 3.8.3"
-  spec.add_development_dependency "webrick", "~> 1.5"
+  spec.add_development_dependency "bundler"
+  spec.add_development_dependency "pry"
+  spec.add_development_dependency "rake"
+  spec.add_development_dependency "rspec"
+  spec.add_development_dependency "yard"
+  spec.add_development_dependency "webmock"
+  spec.add_development_dependency "webrick"
 end