Address deprecation warning from PyYAML

Using `yaml.load()` is considered deprecated and unsafe. See the PyYAML
wiki [0] for more details.

[0]: https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

concourse/pipelines/gen_pipeline.py

@@ -123,7 +123,7 @@ def validate_pipeline_release_jobs(raw_pipeline_yml):
 
     # ignore concourse v2.x variable interpolation
     pipeline_yml_cleaned = re.sub('{{', '', re.sub('}}', '', raw_pipeline_yml))
-    pipeline = yaml.load(pipeline_yml_cleaned)
+    pipeline = yaml.safe_load(pipeline_yml_cleaned)
 
     jobs_raw = pipeline['jobs']
     all_job_names = [job['name'] for job in jobs_raw]