R1:f27f7dce528e

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…