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.

contracts/drafts/Counters.sol

@@ -0,0 +1,24 @@
+pragma solidity ^0.5.2;
+
+/**
+ * @title Counters
+ * @author Matt Condon (@shrugs)
+ * @dev Provides an incrementing uint256 id acquired by the `Counter#next` getter.
+ * Use this for issuing ERC721 ids or keeping track of request ids, anything you want, really.
+ *
+ * Include with `using Counters` for Counters.Counter;`
+ * @notice Does not allow an Id of 0, which is popularly used to signify a null state in solidity.
+ * Does not protect from overflows, but if you have 2^256 ids, you have other problems.
+ * (But actually, it's generally impossible to increment a counter this many times, energy wise
+ * so it's not something you have to worry about.)
+ */
+library Counters {
+    struct Counter {
+        uint256 current; // default: 0
+    }
+
+    function next(Counter storage index) internal returns (uint256) {
+        index.current += 1;
+        return index.current;
+    }
+}