Crowdsale.buyTokens is now nonReentrant. (#1438)

(cherry picked from commit 6d415c508b)

contracts/utils/ReentrancyGuard.sol

@@ -19,11 +19,10 @@ contract ReentrancyGuard {
 
   /**
    * @dev Prevents a contract from calling itself, directly or indirectly.
-   * If you mark a function `nonReentrant`, you should also
-   * mark it `external`. Calling one `nonReentrant` function from
-   * another is not supported. Instead, you can implement a
-   * `private` function doing the actual work, and an `external`
-   * wrapper marked as `nonReentrant`.
+   * Calling a `nonReentrant` function from another `nonReentrant`
+   * function is not supported. It is possible to prevent this from happening
+   * by making the `nonReentrant` function external, and make it call a
+   * `private` function that does the actual work.
    */
   modifier nonReentrant() {
     _guardCounter += 1;