Add JS specs for Array extensions

spec/javascripts/extensions/array_spec.js.coffee

+#= require extensions/array
+
+describe 'Array extensions', ->
+  describe 'first', ->
+    it 'returns the first item', ->
+      arr = [0, 1, 2, 3, 4, 5]
+      expect(arr.first()).toBe(0)
+
+  describe 'last', ->
+    it 'returns the last item', ->
+      arr = [0, 1, 2, 3, 4, 5]
+      expect(arr.last()).toBe(5)