DOC: Update README to reference Ruby 2.3.1
DOC: Update README to reference Ruby 2.3.1

jakerenzella/doubtfire-api,doubtfire-lms/doubtfire-api,final-year-project/doubtfire-api,final-year-project/doubtfire-api,alexcu/doubtfire-api,jakerenzella/doubtfire-api,final-year-project/doubtfire-api,alexcu/doubtfire-api,doubtfire-lms/doubtfire-api,doubtfire-lms/doubtfire-api,jakerenzella/doubtfire-api,jakerenzella/doubtfire-api,final-year-project/doubtfire-api,final-year-project/doubtfire-api,doubtfire-lms/doubtfire-api,doubtfire-lms/doubtfire-api,doubtfire-lms/doubtfire-api,jakerenzella/doubtfire-api,alexcu/doubtfire-api,alexcu/doubtfire-api,doubtfire-lms/doubtfire-api,jakerenzella/doubtfire-api,alexcu/doubtfire-api,jakerenzella/doubtfire-api,alexcu/doubtfire-api