add connect-redis for redis support (#183)

Redis is an optional dependency of Grist. When available, it can
be used for session storage, for supporting a pool of workers, and
for managing webhook delivery. This commit adds connect-redis to
package.json to simplify enabling Redis for the end user.

package.json

@@ -95,6 +95,7 @@
     "brace": "0.11.1",
     "collect-js-deps": "^0.1.1",
     "components-jqueryui": "1.12.1",
+    "connect-redis": "3.4.0",
     "cookie-parser": "1.4.3",
     "csv": "4.0.0",
     "diff-match-patch": "1.0.5",