Fix Pavelib prereqs.

Do not read file as bytes, as it would fail the comparison of byte with str.
PROD-1322

pavelib/prereqs.py

@@ -103,7 +103,7 @@ def prereq_cache(cache_name, paths, install_func):
     cache_file_path = os.path.join(PREREQS_STATE_DIR, "{}.sha1".format(cache_filename))
     old_hash = None
     if os.path.isfile(cache_file_path):
-        with io.open(cache_file_path, "rb") as cache_file:
+        with io.open(cache_file_path, "r") as cache_file:
             old_hash = cache_file.read()
 
     # Compare the old hash to the new hash