Add rubygem-mime-types package from EPEL.

current mime-types from gem raised requirements and need ruby >= 1.9.2
which is not available on CentOS6.
On CentOS7 the gem installed mime-types 2.3 throws
"Encoding::CompatibilityError"s.
So in both cases use the package from EPEL which just works fine for
CentOS6 and CentOS7.