Update changelog for 1.8.1 release

Author: rnorth

CHANGELOG.md

@@ -1,7 +1,7 @@
 # Change Log
 All notable changes to this project will be documented in this file.
 
-## UNRELEASED
+## [1.8.1] - 2018-07-10
 ### Fixed
 - Linux/Mac: Added support for docker credential helpers so that images may be pulled from private registries. See [\#729](https://github.com/testcontainers/testcontainers-java/issues/729), [\#647](https://github.com/testcontainers/testcontainers-java/issues/647) and [\#567](https://github.com/testcontainers/testcontainers-java/issues/567).
 - Ensure that the `COMPOSE_FILE` environment variable is populated with all relevant compose file names when running docker-compose in local mode [\#755](https://github.com/testcontainers/testcontainers-java/issues/755).