Update v10.6.0 changelog

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CHANGELOG.md

-v 10.6.0-rc2 (2018-03-21)
+v 10.6.0 (2018-03-22)
 - Upgrade docker-machine to v0.14.0 !850
 - Upgrade helper image to alpine 3.6 !859
-
-v 10.6.0-rc1 (2018-03-08)
 - Add CI_RUNNER_VERSION, CI_RUNNER_REVISION, and CI_RUNNER_EXECUTABLE_ARCH job environment variables !788
 - Always prefer creating new containers when running with Docker Executor !818
 - Use IAM instance profile credentials for S3 caching !646