Merge branch 'release-v1.12.0'

ethpm.json

 {
   "package_name": "zeppelin",
-  "version": "1.11.0",
+  "version": "1.12.0",
   "description": "Secure Smart Contract library for Solidity",
   "authors": [
     "OpenZeppelin Community <maintainers@openzeppelin.org>"

package-lock.json

 {
   "name": "openzeppelin-solidity",
-  "version": "1.11.0",
+  "version": "1.12.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {

package.json

 {
   "name": "openzeppelin-solidity",
-  "version": "1.11.0",
+  "version": "1.12.0",
   "description": "Secure Smart Contract library for Solidity",
   "files": [
     "contracts",