Pin prospector version to 1.1.6.2

prospector 1.1.6.4 is currently broken - it updates to a new version of
pylint that it isn't compatible with. The relevant issue is here:
https://github.com/PyCQA/prospector/issues/335

tildes/requirements-to-freeze.txt

@@ -14,7 +14,7 @@ mypy==0.670  # 0.700 includes incorrect bleach stubs
 mypy-extensions
 Pillow
 prometheus-client
-prospector
+prospector==1.1.6.2
 psycopg2
 publicsuffix2==2.20160818
 pygit2