Added ruby 2.3 install on centos 7

net-telnet and puma both require Ruby to be over 2.2.

Either you install the old variants of those 2, or update ruby to 2.3

README.md

@@ -111,6 +111,13 @@ RHEL 7 / CentOS 7 will work out of the box with the following package list:
 ```shell
 yum install make cmake which sqlite-devel openssl-devel libssh2-devel ruby gcc ruby-devel libicu-devel gcc-c++
 ```
+Then you have to install at least Ruby 2.3 for for the ruby requirements to work:
+
+```shell
+yum install centos-release-scl
+yum install rh-ruby22
+scl enable rh-ruby22 bash
+```
 
 Now let's install oxidized via Rubygems: