Removes yum-config-manager from centos reqs, EPEL comes enabled

**Problem**: `yum-config-manager` is not installed on CentOS minimal and the rvm install fails with:

    Error running 'requirements_centos_update_system ruby-1.9.3-p547',
    showing last 15 lines of /home/user/.rvm/log/1407555224_ruby-1.9.3-p547/update_system.log
    ++ /scripts/functions/logging : rvm_pretty_print()  81 > case "$1" in
    ++ /scripts/functions/logging : rvm_pretty_print()  83 > [[ -t 2 ]]
    ++ /scripts/functions/logging : rvm_pretty_print()  83 > return 1
    ++ /scripts/functions/logging : rvm_error()  117 > printf %b 'Error installing EPEL, it is required for libyaml-devel,
    either there was an error installing EPEL package,
    or there was problem checking if libyaml-devel is available / installed.\n'
    Error installing EPEL, it is required for libyaml-devel,
    either there was an error installing EPEL package,
    or there was problem checking if libyaml-devel is available / installed.
    ++ /scripts/functions/requirements/centos : requirements_centos_update_system_install_epel()  41 > return 1
    ++ /scripts/functions/utility_logging : __rvm_log_command_simple()  83 > return 1
    ++ /scripts/functions/utility_logging : __rvm_log_command()  61 > return 1
    ++ /scripts/functions/build_requirements_helpers : rvm_requiremnts_fail_or_run_action()  158 > return 1
    ++ /scripts/functions/requirements/centos : requirements_centos_update_system_check_epel()  66 > return 1
    ++ /scripts/functions/requirements/centos : requirements_centos_update_system()  72 > return 1
    Requirements installation failed with status: 1.
**Reasoning**: Installing EPEL enables it by default, there's no need to enable it again. This was tested on CentOS 5:

    $ cat /etc/issue
    CentOS release 5.7 (Final)
    Kernel \r on an \m
    $ rpm -qa | grep epel
    $ sudo rpm -ivh http://mirror.steadfast.net/epel/5/i386/epel-release-5-4.noarch.rpm
    Retrieving http://mirror.steadfast.net/epel/5/i386/epel-release-5-4.noarch.rpm
    Preparing...                ########################################### [100%]
       1:epel-release           ########################################### [100%]
    $ yum repolist enabled | grep epel
    epel            Extra Packages for Enterprise Linux 5 - x86_64            7,739
And 6:

    $ cat /etc/issue
    CentOS release 6.5 (Final)
    Kernel \r on an \m
    $ rpm -qa | grep epel
    $ sudo rpm -ivh http://fedora.mirrors.pair.com/epel/6/i386/epel-release-6-8.noarch.rpm
    Retrieving http://fedora.mirrors.pair.com/epel/6/i386/epel-release-6-8.noarch.rpm
    Preparing...                ########################################### [100%]
       1:epel-release           ########################################### [100%]
    $ yum repolist enabled | grep epel
     * epel: mirror.prgmr.com
    *epel            Extra Packages for Enterprise Linux 6 - x86_64           11,082
**Solution**: :ship: this PR :smile: