Generate changelog without v prefix

This repo historically used v1.x.x tags, but we stopped tagging with the `v`
prefix in 1.8.2. However, when doing the 1.8.14 release, both 1.8.14 and v1.8.14
tags were created. So this commit excludes the v1.8.14 tag to avoid creating an
empty changelog diff for `v1.8.14...1.8.14`

And it updates the Rakefile to not prefix the future tag with `v`.

Author: joshcooper

Rakefile

@@ -51,12 +51,13 @@ begin
   require 'github_changelog_generator/task'
   GitHubChangelogGenerator::RakeTask.new :changelog do |config|
     require 'puppet/resource_api/version'
-    config.future_release = "v#{Puppet::ResourceApi::VERSION}"
+    config.future_release = "#{Puppet::ResourceApi::VERSION}"
     config.header = "# Changelog\n\n" \
       "All significant changes to this repo will be summarized in this file.\n"
     # config.include_labels = %w[enhancement bug]
     config.user = 'puppetlabs'
     config.project = 'puppet-resource_api'
+    config.exclude_tags = ['v1.8.14']
   end
 rescue LoadError
   desc 'Install github_changelog_generator to get access to automatic changelog generation'