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/PausableMock.sol

@@ -9,7 +9,7 @@ contract PausableMock is Pausable, PauserRoleMock {
     uint256 public count;
 
     constructor () public {
-        Pausable.initialize(msg.sender);
+        Pausable.initialize(_msgSender());
 
         drasticMeasureTaken = false;
         count = 0;