update version number in README.md

Author: horgh

README.md

@@ -18,7 +18,7 @@ To do this, add the dependency to your pom.xml:
     <dependency>
         <groupId>com.maxmind.geoip2</groupId>
         <artifactId>geoip2</artifactId>
-        <version>2.15.0-rc1</version>
+        <version>2.15.0</version>
     </dependency>
 ```
 
@@ -31,7 +31,7 @@ repositories {
     mavenCentral()
 }
 dependencies {
-    compile 'com.maxmind.geoip2:geoip2:2.15.0-rc1'
+    compile 'com.maxmind.geoip2:geoip2:2.15.0'
 }
 ```