update README.md

Author: luciuschoi

README.md

@@ -73,7 +73,8 @@ Among the gem sources, a test application is provided and there you can find how
   - v 3.3.0.1 : deployed to Rubygems.org
   - v 3.3.0.2 : modified @import stylesheet syntax
   - v 3.3.0.3 : fixed typos in ekko-lightbox.min.js and ekko-lightbox.js ([lightbox v4.0.2](https://github.com/ashleydw/lightbox/tree/v4.0.2) for Boostrap 3).
-  - v 5.0.0.0 : updated with [lightbox v5.0.0](https://github.com/ashleydw/lightbox/tree/v5.0.0) for Boostrap 4.
+  - v 5.0.0.0 : updated with [lightbox v5.0.0](https://github.com/ashleydw/lightbox/tree/v5.0.0) for Boostrap 4 alpha 5
+  - v 5.1.0.0 : updated with [lightbox v5.1.0](https://github.com/ashleydw/lightbox/tree/v5.1.0) for Boostrap 4 alpha 6
 
 ## Contributing