gotipplay: fix GOTIP environment variable to be string

Change-Id: If2aca632dd49cc161e7034d7231937b53e18c592
Reviewed-on: https://go-review.googlesource.com/c/playground/+/366179
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

Author: findleyr

app.gotip.yaml

@@ -18,5 +18,5 @@ readiness_check:
 
 env_variables:
   MEMCACHED_ADDR: 'memcached-play-golang:11211'
-  GOTIP: true
+  GOTIP: "true"