Add support for multiple ruby versions

This commit adds support for multiple ruby versions, now our CI
will run our test suite for each of the major ruby version. The
version we supporting so far are 2.1.9, 2.2.0, 2.3.1 and 2.4.1

.travis.yml

@@ -1,5 +1,8 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.3.3
-before_install: gem install bundler -v 1.14.3
+  - 2.4.1
+  - 2.3.1
+  - 2.2.0
+  - 2.1.9
+before_install: gem install bundler -v 1.14.6

README.md

@@ -1,6 +1,11 @@
 # Digicert
 
-[![CircleCI](https://circleci.com/gh/abunashir/digicert-api/tree/master.svg?style=svg&circle-token=21edcc7e3704f7d5b689e87b9e3af658e34a5be4)](https://circleci.com/gh/abunashir/digicert-api/tree/master)
+[![Build
+Status](https://travis-ci.org/riboseinc/digicert.svg?branch=master)](https://travis-ci.org/riboseinc/digicert)
+[![Code
+Climate](https://codeclimate.com/github/riboseinc/digicert/badges/gpa.svg)](https://codeclimate.com/github/riboseinc/digicert)
+
+The Ruby client for the official Digicert API.
 
 ## Installation