Update README.md

Author: aarongoin

README.md

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 # Java Client Library
 
-* Latest released version 0.15.3
-* Latest snapshot version 0.15.4-SNAPSHOT
+* Latest released version 0.15.4
+* Latest snapshot version 0.15.5-SNAPSHOT
 
 ## Introduction
 This is the PolyAPI Java client GitHub page. If you are here, then it means you're familiar with what we do at Poly. If you aren't, you can always check [here](https://github.com/polyapi/poly-alpha).