Fix compiler warning on concatenation

[tedyu]

Noticed the following warning in build output:

/Users/tedyu/cassandra-cpp-driver/test/ccm_bridge/src/bridge.cpp:1879:86: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
    throw BridgeException("No Nodes are Available: Limiting total nodes for CCM to " + CLUSTER_NODE_LIMIT);
                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~