fix to truffle 4.1.5 only

Author: fbranciard

package.json

@@ -31,7 +31,7 @@
     "replace": "^0.3.0",
     "solidity-coverage": "^0.4.12",
     "solidity-sha3": "0.4.1",
-    "truffle": "^4.1.5",
+    "truffle": "4.1.5",
     "truffle-contract": "3.0.4",
     "truffle-hdwallet-provider": "0.0.3",
     "web3": "0.20.2"