Remove official support for Drupal 8.3 and Drush9 as Drupal itself do…

…esn't support 8.3 anymore

docs/install.md

@@ -10,7 +10,7 @@ Install a site-local Drush and Drush Launcher.
 1. To be able to call `drush` from anywhere, install the [Drush Launcher](https://github.com/drush-ops/drush-launcher). That is a small program which listens on your $PATH and hands control to a site-local Drush that is in the /vendor directory of your Composer project. If you skip this step, run Drush from Drupal root via `../vendor/bin/drush`. In that case Drush's bash integration and custom prompt won't work.
 1. Run `drush init`. This edits ~/.bashrc so that Drush's custom prompt and bash integration are active.
 1. See [Usage](http://docs.drush.org/en/master/usage/) for details on using Drush.
-1. To use a non-default PHP, [edit ~/.bashrc so that the desired PHP is in front of your $PATH](http://stackoverflow.com/questions/4145667/how-to-override-the-path-of-php-to-use-the-mamp-path/10653443#10653443). If that is not desireable, you can change your PATH for just one request: `PATH=/path/to/php:$PATH` drush status ...`
+1. To use a non-default PHP, [edit ~/.bashrc so that the desired PHP is in front of your $PATH](http://stackoverflow.com/questions/4145667/how-to-override-the-path-of-php-to-use-the-mamp-path/10653443#10653443). If that is not desirable, you can change your PATH for just one request: `PATH=/path/to/php:$PATH` drush status ...`
 
 !!! note
 
@@ -20,7 +20,7 @@ Drupal Compatibility
 -----------------
 Drush Version | Drush Branch  | PHP | Compatible Drupal versions | Code Status
 ------------- | ---------     | --- | -------------------------- | -----------
-Drush 9       | [master](https://travis-ci.org/drush-ops/drush)  | 5.6+ | D8.3+                    | <img src="https://travis-ci.org/drush-ops/drush.svg?branch=master">
+Drush 9       | [master](https://travis-ci.org/drush-ops/drush)  | 5.6+ | D8.4+                    | <img src="https://travis-ci.org/drush-ops/drush.svg?branch=master">
 Drush 8       | [8.x](https://travis-ci.org/drush-ops/drush)  | 5.4.5+ | D6, D7, D8.3-             | <img src="https://travis-ci.org/drush-ops/drush.svg?branch=8.x">
 Drush 7       | [7.x](https://travis-ci.org/drush-ops/drush) | 5.3.0+ | D6, D7                     | Unsupported
 Drush 6       | [6.x](https://travis-ci.org/drush-ops/drush) | 5.3.0+ | D6, D7                     | Unsupported