Remove redundant kernel check in AragonApp.getRecoveryVault() (#433)

* Remove redundant checks before making calls

When using the native call syntax `target.method(...)` solc introduces
a check for the code size of the target (and reverts if it is 0), therefore
we don't need to manually ensure that the target address is not 0 before making
a call, as the size of the address 0 will be 0, and therefore already revert

* Revert removing ACL existence check in kernel.hasPermission

contracts/apps/AragonApp.sol

@@ -62,7 +62,6 @@ contract AragonApp is AppStorage, Autopetrified, VaultRecoverable, EVMScriptRunn
     */
     function getRecoveryVault() public view returns (address) {
         // Funds recovery via a vault is only available when used with a kernel
-        require(address(kernel()) != address(0));
-        return kernel().getRecoveryVault();
+        return kernel().getRecoveryVault(); // if kernel is not set, it will revert
     }
 }