Issue setup guidance recommending MySQLi and MySQL Native Driver

Summary:
Fixes T12994. We need `MYSQLI_ASYNC` to implement client-side query timeouts, and we need MySQLi + MySQL Native Driver to get `MYSQLI_ASYNC`.

Recommend users install MySQLi and MySQL Native Driver if they don't have them. These are generally the defaults and best practice anyway, but Ubuntu makes it easy to use the older stuff.

All the cases we're currently aware of stem from `apt-get install php5-mysql` (which explicitly selects the non-native driver) so issue particular guidance about `php5-mysqlnd`.

Test Plan:
  - Faked both issues locally, reviewed the text.
  - Will deploy to `secure`, which currently has the non-native driver.

Maniphest Tasks: T12994

Differential Revision: https://secure.phabricator.com/D19216

scripts/install/install_ubuntu.sh

@@ -45,7 +45,7 @@ set +x
 sudo apt-get -qq update
 sudo apt-get install \
   $GIT mysql-server apache2 dpkg-dev \
-  php5 php5-mysql php5-gd php5-dev php5-curl php-apc php5-cli php5-json
+  php5 php5-mysqlnd php5-gd php5-dev php5-curl php-apc php5-cli php5-json
 
 # Enable mod_rewrite
 sudo a2enmod rewrite