remove note about 2.0 being experimental (#1565)

(cherry picked from commit 5036741)

Author: frangio

README.md

@@ -12,8 +12,6 @@
 npm install openzeppelin-solidity
 ```
 
-If you're interested in trying out a preview of OpenZeppelin 2.0, install `openzeppelin-solidity@next`, check out the [release notes](https://github.com/OpenZeppelin/openzeppelin-solidity/releases/tag/v2.0.0-rc.1), and let us know what you think!
-
 ## Usage
 
 To write your custom contracts, import ours and extend them through inheritance.