Move hash_reqs.py from tools/ to scripts/lib/.

This is needed because hash_reqs.py is used to create a virtualenv.
Currently we only use virtualenv in development, but we will soon
start using it in production.  Scripts used in production should be
put in scripts/.

scripts/lib/setup_venv.py

@@ -21,7 +21,7 @@ def setup_virtualenv(target_venv_path, requirements_file, virtualenv_args=None):
     # type: (str, str, List[str]) -> None
 
     # Check if a cached version already exists
-    path = os.path.join(ZULIP_PATH, 'tools', 'hash_reqs.py')
+    path = os.path.join(ZULIP_PATH, 'scripts', 'lib', 'hash_reqs.py')
     output = subprocess.check_output([path, requirements_file])
     sha1sum = output.split()[0]
     cached_venv_path = os.path.join(VENV_CACHE_PATH, sha1sum, os.path.basename(target_venv_path))