Upgrade to Rxjava2 2.2.18

Closes gh-20321

Author: snicoll

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

@@ -156,7 +156,7 @@
 		<reactive-streams.version>1.0.3</reactive-streams.version>
 		<rxjava.version>1.3.8</rxjava.version>
 		<rxjava-adapter.version>1.2.1</rxjava-adapter.version>
-		<rxjava2.version>2.2.17</rxjava2.version>
+		<rxjava2.version>2.2.18</rxjava2.version>
 		<selenium.version>3.14.0</selenium.version>
 		<selenium-htmlunit.version>2.33.0</selenium-htmlunit.version>
 		<sendgrid.version>4.3.0</sendgrid.version>