Specify exact version in generator

If the React on Rails version is standard, then that version is used
to specify the npm package to load.

If the React on Rails version is a beta version, then no version is
specified.

lib/generators/react_on_rails/base_generator.rb

@@ -43,7 +43,14 @@ def add_base_gems_to_gemfile
       end
 
       def add_yarn_dependencies
-        run "yarn add react-on-rails --exact"
+        major_minor_patch_only = /\A\d+\.\d+\.\d+\z/.freeze
+        if ReactOnRails::VERSION.match?(major_minor_patch_only)
+          run "yarn add react-on-rails@#{ReactOnRails::VERSION} --exact"
+        else
+          # otherwise add latest
+          puts "Adding the lastest react-on-rails NPM module. Double check this is correct in package.json"
+          run "yarn add react-on-rails --exact"
+        end
       end
 
       def append_to_spec_rails_helper