Install PHP mbstring extension on RHEL & friends

Summary:
The mbstring extension for PHP is not a dependency to any of the already
listed packages on RHEL-like systems, and is needed by Phabricator
(showing a "install mbstring" message as the first thing if it is not
installed)

RHEL seems to have some extra steps to allow php-mbstring to be installed, though:

http://snippets.roozbehk.com/post/35750940300/php-mbstring-missing-on-red-hat-enterprise-linux-6

PS: disabled lint for this change because of the already >80 chars long "yum install" string

Test Plan:
* Created a new container with docker using both centos:6.4 and fedora:20 images
* Ran install script
* Started httpd and mysqld services
* Browsed to server's address
* Got error message
* Installed php-mbstring & restarted httpd
* Works

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8772