Fix warnings (#1606)

* Bump required compiler version to 0.5.2.

* Fix shadowed variable warning in ERC20Migrator.

* Rename Counter to Counters.

* Add dummy state variable to SafeERC20Helper.

* Update changelog entry.

* Fix CountersImpl name.

* Improve changelog entry.

truffle-config.js

@@ -16,7 +16,7 @@ module.exports = {
 
   compilers: {
     solc: {
-      version: '0.5.0',
+      version: '0.5.2',
     },
   },
 };