Update flatland-rl to 2.1.6

environment.yml

@@ -71,7 +71,7 @@ dependencies:
     - crowdai-api==0.1.21
     - cycler==0.10.0
     - filelock==3.0.12
-    - flatland-rl==2.1.5
+    - flatland-rl==2.1.6
     - future==0.17.1
     - gym==0.14.0
     - idna==2.8