Make CouchDB align with upstream version. (#156)

Bump couchdb image into apache/couchdb:2.1. Since couchdb 1 vs 2 is not 100% compatible that might cause potential problems if any of upstream changes.
diff --git a/docker-compose/docker-compose.yml b/docker-compose/docker-compose.yml
index 3006fa0..6d39fce 100644
--- a/docker-compose/docker-compose.yml
+++ b/docker-compose/docker-compose.yml
@@ -1,7 +1,7 @@
 version: '3'
 services:
   db:
-    image: couchdb:1.6
+    image: apache/couchdb:2.1
     ports:
       - "5984:5984"
     environment: