Bump pyyaml version in conda environment.

Fixes CVE in yaml/pyyaml#168.

environment.yml

@@ -12,7 +12,7 @@ dependencies:
   - pycodestyle=2.4.0 # Dev dependency.
   - pylint=1.9.2 # Dev dependency.
   - pip:
-      - pyyaml==4.2b1
+      - pyyaml>=5.1
       - responses==0.10.2 # Dev dependency.
       - schedule==0.5.0
       - httplib2shim==0.0.3