add responsive CSS to hyperlinked images when thumbnail caption markup is used

When thumbnail caption markup is used in combination with a hyperlinked image, the image is not responsive. (non-hyperlinked images are fine, like the example in the docs) Adding a selector to apply responsiveness to hyperlinked images when thumbnail captions are used.

less/thumbnails.less

@@ -9,7 +9,8 @@
   display: block; // Override the inline-block from `.img-thumbnail`
   margin-bottom: @line-height-computed;
 
-  > img {
+  > img,
+  a > img {
     .img-responsive();
   }
 }