Upgrade to Rxjava2 2.1.9

Closes gh-11945

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -132,7 +132,7 @@
 		<reactive-streams.version>1.0.2</reactive-streams.version>
 		<rxjava.version>1.3.5</rxjava.version>
 		<rxjava-adapter.version>1.2.1</rxjava-adapter.version>
-		<rxjava2.version>2.1.8</rxjava2.version>
+		<rxjava2.version>2.1.9</rxjava2.version>
 		<selenium.version>3.8.1</selenium.version>
 		<selenium-htmlunit.version>2.29.0</selenium-htmlunit.version>
 		<sendgrid.version>4.1.2</sendgrid.version>