updates README with current version 0.20

Chris Wilson · Chris Wilson · commit e1495221d194 · 2018-03-02T15:19:54.000-06:00
diff --git a/README.md b/README.md
@@ -27,7 +27,7 @@ The SparkPost Java Library is available in this [Maven Repository](https://repo.
 <dependency>
 	<groupId>com.sparkpost</groupId>
 	<artifactId>sparkpost-lib</artifactId>
-	<version>0.19</version>
+	<version>0.20</version>
 </dependency>
 ```
 
