install_deps supports updates

Summary:
When contributing to pytext, fetching the pip dependencies and
keeping them up to date is not documented. This script can now fetch the
initial dependencies, and when re-running afterwards it will update them.

Differential Revision: D16572679

fbshipit-source-id: fe3b8be0fde88eb9a3187d8ee5590f972dfc3b6f

Author: Titousensei

install_deps

@@ -1,3 +1,4 @@
 #!/usr/bin/env bash
 pip install --upgrade pip
-pip install -e . --upgrade --no-cache-dir --progress-bar off
+pip install --upgrade torch
+pip install -e . --upgrade --no-cache-dir --progress-bar off --upgrade-strategy eager