add REQUEST_TIME_FLOAT to $_SERVER

Tested on my repo

[root@localhost ~]# hhvm server.php  | grep REQUEST
    [SELINUX_ROLE_REQUESTED] => 
    [SELINUX_LEVEL_REQUESTED] => 
    [REQUEST_START_TIME] => 1368169462
    [REQUEST_TIME] => 1368169462
    [REQUEST_TIME_FLOAT] => 1368169462.4599

EL6 rpm http://www.hop5.in/yum/el6/hiphop-php-2.1.210-1.el6.x86_64.rpm