Create conversion map

tcr-roman-numerals/src/main/java/fr/ippon/kata/numeral/Numerals.java

 package fr.ippon.kata.numeral;
 
+import java.util.NavigableMap;
+import java.util.TreeMap;
+
 public class Numerals {
+  private static final NavigableMap<Integer, String> CONVERSIONS = buildConversions();
 
   public static String toRoman(int arabic) {
     if (arabic >= 5) {
@@ -13,4 +17,14 @@ public class Numerals {
 
     return "I".repeat(arabic);
   }
+
+  private static NavigableMap<Integer, String> buildConversions() {
+    NavigableMap<Integer, String> conversions = new TreeMap<>();
+
+    conversions.put(1, "I");
+    conversions.put(4, "IV");
+    conversions.put(5, "V");
+
+    return conversions;
+  }
 }