Change ECRecovery recover method's visibility to internal (#818)

* Change ECRecovery recover method's visibility to internal

* Adopt the test

* Remove unused imports

contracts/ECRecovery.sol

@@ -14,7 +14,7 @@ library ECRecovery {
    * @param hash bytes32 message, the hash is the signed message. What is recovered is the signer address.
    * @param sig bytes signature, the signature is generated using web3.eth.sign()
    */
-  function recover(bytes32 hash, bytes sig) public pure returns (address) {
+  function recover(bytes32 hash, bytes sig) internal pure returns (address) {
     bytes32 r;
     bytes32 s;
     uint8 v;

test/library/ECRecovery.test.js

@@ -1,5 +1,4 @@
 var ECRecoveryMock = artifacts.require('ECRecoveryMock');
-var ECRecoveryLib = artifacts.require('ECRecovery');
 
 var hashMessage = require('../helpers/hashMessage.js');
 
@@ -8,8 +7,6 @@ contract('ECRecovery', function (accounts) {
   const TEST_MESSAGE = 'OpenZeppelin';
 
   before(async function () {
-    const ecRecoveryLib = await ECRecoveryLib.new();
-    ECRecoveryMock.link('ECRecovery', ecRecoveryLib.address);
     ecrecovery = await ECRecoveryMock.new();
   });