GSN compatibility (#1880)

* switch to using Context internally

* add context import

* Add smoke test to make sure enabling GSN support works

* Update test/GSN/ERC721GSNRecipientMock.test.js

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Upgrade truffle

* add missing awaits

* Revert "Upgrade truffle"

This reverts commit f9b0ba9019.

contracts/mocks/ERC721GSNRecipientMock.sol

@@ -0,0 +1,18 @@
+pragma solidity ^0.5.0;
+
+import "../token/ERC721/ERC721.sol";
+import "../GSN/GSNRecipient.sol";
+import "../GSN/bouncers/GSNBouncerSignature.sol";
+
+/**
+ * @title ERC721GSNRecipientMock
+ * A simple ERC721 mock that has GSN support enabled
+ */
+contract ERC721GSNRecipientMock is ERC721, GSNRecipient, GSNBouncerSignature {
+    constructor(address trustedSigner) public GSNBouncerSignature(trustedSigner) { }
+    // solhint-disable-previous-line no-empty-blocks
+
+    function mint(uint256 tokenId) public {
+        _mint(_msgSender(), tokenId);
+    }
+}