Prepare for v5.15 release

Author: kmanning

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+# v5.15
+
 * [Issue #172](https://github.com/manheim/terraform-pipeline/issues/172) Feature: ConditionalApplyPlugin - can allow apply for specific environments on all branches/PRs.
 * [Issue #329](https://github.com/manheim/terraform-pipeline/issues/329) Feature: ConditionalApplyPlugin - can be disabled to allow apply on all branches/PRs.
 * [Issue #320](https://github.com/manheim/terraform-pipeline/issues/320) Bug Fix: GithubPRPlugin should stop on plan errors, and display error to the user.