Widen the pragma (#71)

* Widen the pragma

This reverts commit 528056ed

contracts/interfaces/IERC20.sol

@@ -1,4 +1,4 @@
-pragma solidity =0.5.16;
+pragma solidity >=0.5.0;
 
 interface IERC20 {
     event Approval(address indexed owner, address indexed spender, uint value);

contracts/interfaces/IUniswapV2Callee.sol

@@ -1,4 +1,4 @@
-pragma solidity =0.5.16;
+pragma solidity >=0.5.0;
 
 interface IUniswapV2Callee {
     function uniswapV2Call(address sender, uint amount0, uint amount1, bytes calldata data) external;

contracts/interfaces/IUniswapV2ERC20.sol

@@ -1,4 +1,4 @@
-pragma solidity =0.5.16;
+pragma solidity >=0.5.0;
 
 interface IUniswapV2ERC20 {
     event Approval(address indexed owner, address indexed spender, uint value);

contracts/interfaces/IUniswapV2Factory.sol

@@ -1,4 +1,4 @@
-pragma solidity =0.5.16;
+pragma solidity >=0.5.0;
 
 interface IUniswapV2Factory {
     event PairCreated(address indexed token0, address indexed token1, address pair, uint);

contracts/interfaces/IUniswapV2Pair.sol

@@ -1,4 +1,4 @@
-pragma solidity =0.5.16;
+pragma solidity >=0.5.0;
 
 interface IUniswapV2Pair {
     event Approval(address indexed owner, address indexed spender, uint value);