force zepplein version for travis

Author: jrainville

test_apps/test_app/package.json

@@ -19,6 +19,6 @@
     "react": "^16.0.0",
     "react-bootstrap": "^0.32.0",
     "react-dom": "^16.2.0",
-    "zeppelin-solidity": "^1.8.0"
+    "zeppelin-solidity": "1.8.0"
   }
 }