update image-gallery to 2.10

I don't know why it's incompatible, but with jquery-rails 2.1.4 and bootstrap-sass 2.2.2.0 image-gallery 2.8 doesn't work and firebug doesn't show any error.

From the patch I guess that could be something related to this, but I don't know:
-                    options.selector = 'a[rel=gallery]';
+                    options.selector = 'a[data-gallery=gallery]';

With this patch I could make it work with the instructions in README.