example-cluster: drop down cluster version

After cartridge 2.7.7, you don't bind to 0.0.0.0 by default anymore [1].
In consequence, other containers and host fails to connect with IProto.
Explicit `0.0.0.0` and `127.0.0.1` don't help to fix the issue.

1. tarantool/cartridge#1890

Author: DifferentialOrange

example_cluster/project/project-scm-1.rockspec

@@ -7,7 +7,7 @@ source  = {
 dependencies = {
     'tarantool',
     'lua >= 5.1',
-    'cartridge == 2.7.8-1',
+    'cartridge == 2.7.6-1',
     'metrics == 0.16.0-1',
     'crud == 1.0.0-1',
     'expirationd == 1.3.1-1',