Update to use class methods

exercises/practice/resistor-color-duo/.meta/example.coffee

@@ -1,5 +1,5 @@
 class ResistorColorDuo
-  colorBands: [
+  @colorBands: [
         "black"
         "brown"
         "red"
@@ -12,7 +12,7 @@ class ResistorColorDuo
         "white"
     ]
 
-  value: ([first, second]) ->
+  @value: ([first, second]) ->
    10 * @colorBands.indexOf(first) + @colorBands.indexOf(second)
 
 module.exports = ResistorColorDuo

exercises/practice/resistor-color-duo/resistor-color-duo.coffee

@@ -1,4 +1,4 @@
 class ResistorColorDuo
-  value: (colors) ->
+  @value: (colors) ->
 
 module.exports = ResistorColorDuo

exercises/practice/resistor-color-duo/resistor-color-duo.spec.coffee

@@ -1,38 +1,37 @@
 ResistorColorDuo = require './resistor-color-duo'
 
 describe 'ResistorColorDuo', ->
-  rc = new ResistorColorDuo()
   it 'Brown and black', ->
-    results = rc.value(['brown', 'black'])
+    results = ResistorColorDuo.value(['brown', 'black'])
     expected = 10
     expect(results).toEqual expected
 
   xit 'Blue and grey', ->
-    results = rc.value(['blue', 'grey'])
+    results = ResistorColorDuo.value(['blue', 'grey'])
     expected = 68
     expect(results).toEqual expected
 
   xit 'Yellow and violet', ->
-    results = rc.value(['yellow', 'violet'])
+    results = ResistorColorDuo.value(['yellow', 'violet'])
     expected = 47
     expect(results).toEqual expected
 
   xit 'White and red', ->
-    results = rc.value(['white', 'red'])
+    results = ResistorColorDuo.value(['white', 'red'])
     expected = 92
     expect(results).toEqual expected
 
   xit 'Orange and orange', ->
-    results = rc.value(['orange', 'orange'])
+    results = ResistorColorDuo.value(['orange', 'orange'])
     expected = 33
     expect(results).toEqual expected
 
   xit 'Ignore additional colors', ->
-    results = rc.value(['green', 'brown', 'orange'])
+    results = ResistorColorDuo.value(['green', 'brown', 'orange'])
     expected = 51
     expect(results).toEqual expected
 
   xit 'Black and brown, one-digit', ->
-    results = rc.value(['black', 'brown'])
+    results = ResistorColorDuo.value(['black', 'brown'])
     expected = 1
     expect(results).toEqual expected