guest: redhat: use yum to install epel-release

Originally the epel-release rpm was not in the main repos for CentOS
and thus we needed to grab them directly from a known url. In el7 they
started putting them in the main repos and I just confirmed that
epel-release is in the main repos for el6 as well so there is no reason
not to use yum now, which will use the mirrors to pull in epel-release.

Fixes #101
Fixes #108

lib/vagrant-sshfs/cap/guest/redhat/sshfs_client.rb

@@ -23,12 +23,7 @@ def self.epel_installed(machine)
         end
 
         def self.epel_install(machine)
-          case machine.guest.capability("flavor")
-            when :rhel_7
-              machine.communicate.sudo("rpm -ivh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm")
-            when :rhel # rhel6
-              machine.communicate.sudo("rpm -ivh https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm")
-          end
+          machine.communicate.sudo("yum -y install epel-release")
         end
       end
     end