fix(progressbar): use more visible color

- Due to the fixed color of the bar, switch to use white text for readability

Closes #4044

src/progressbar/docs/demo.html

@@ -9,7 +9,7 @@ <h3>Static</h3>
 
     <hr />
     <h3>Dynamic <button class="btn btn-sm btn-primary" type="button" ng-click="random()">Randomize</button></h3>
-    <progressbar max="max" value="dynamic"><span style="color:black; white-space:nowrap;">{{dynamic}} / {{max}}</span></progressbar>
+    <progressbar max="max" value="dynamic"><span style="color:white; white-space:nowrap;">{{dynamic}} / {{max}}</span></progressbar>
 
     <small><em>No animation</em></small>
     <progressbar animate="false" value="dynamic" type="success"><b>{{dynamic}}%</b></progressbar>