(SIMP-2117) Clearly specify EPEL installation (#130)

Added a specific `yum install` for the EPEL installation or EL 6 and EL 7

SIMP-2117 #close

docs/getting_started_guide/ISO_Build/Environment_Preparation.rst

@@ -24,12 +24,14 @@ Required RPMs
 
   # Installing from EL 6:
 
+    $ sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
     $ sudo yum install -y augeas-devel createrepo genisoimage git gnupg2 \
         libicu-devel libxml2 libxml2-devel libxslt libxslt-devel \
         mock rpm-sign rpmdevtools clamav gcc gcc-c++ ruby-devel
 
   # Installing from EL 7:
 
+    $ sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
     $ sudo yum install -y augeas-devel createrepo genisoimage git gnupg2 \
         libicu-devel libxml2 libxml2-devel libxslt libxslt-devel \
         mock rpm-sign rpmdevtools clamav-update gcc gcc-c++ ruby-devel