roman.rb gives wrong answer for cases "4: IV, 9: IX"

[Bodin Chinthanet]

File: roman/roman.rb 7d894f8

Case 1

Input: 4
Expected output: "IV"
Actual output: "IIII"

Case 2

Input: 9
Expected output: "IX"
Actual output: "VIIII"

[Starlynne]

Submitted a fix for Ruby Roman
Pull #423