Merge branch 'release-v3.1.0'

CHANGELOG.md

 # Changelog
 
-## 3.1.0 (unreleased)
+## 3.1.0 (2020-06-23)
 
 ### New features
  * `SafeCast`: added functions to downcast signed integers (e.g. `toInt32`), improving usability of `SignedSafeMath`. ([#2243](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2243))

contracts/package.json

 {
   "name": "@openzeppelin/contracts",
-  "version": "3.0.2",
+  "version": "3.1.0",
   "description": "Secure Smart Contract library for Solidity",
   "files": [
     "**/*.sol",

package-lock.json

 {
   "name": "openzeppelin-solidity",
-  "version": "3.0.2",
+  "version": "3.1.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {

package.json

 {
   "name": "openzeppelin-solidity",
-  "version": "3.0.2",
+  "version": "3.1.0",
   "description": "Secure Smart Contract library for Solidity",
   "files": [
     "/contracts/**/*.sol",