Use "public-api" for URI instead of version no

Brightpearl are deprecating the use of specific version numbers in the URI for their public API. Instead, you should use "public-api" in the same location within the URI.

- Change "2.0.0" to "public-api".
- Don't allow this change to be overridden in configuration.

This change will result in the config ignoring API versions being set, so the change is non-breaking.

I have tested the new URI's and they work.

lib/nacre/configuration.rb

@@ -56,7 +56,7 @@ def default_distribution_center
     end
 
     def default_api_version
-      ENV["NACRE_API_VERSION"] || "2.0.0"
+      "public-api"
     end
 
     def default_user_id

spec/lib/nacre/configuration_spec.rb

@@ -2,7 +2,6 @@
 
 describe Nacre::Configuration do
 
-
   let(:config) { Nacre::Configuration.new(args) }
 
   let(:args) { {} }
@@ -31,7 +30,7 @@
     end
 
     it "should have the correct Resource URL" do
-      expect(config.resource_url).to eql("https://ws-eu1.brightpearl.com/2.0.0/%s" % [ENV["NACRE_USER_ID"]])
+      expect(config.resource_url).to eql("https://ws-eu1.brightpearl.com/public-api/%s" % [ENV["NACRE_USER_ID"]])
     end
 
     it "should have the correct auth URL" do