Bump rxjava from 2.2.5 to 2.2.18

Bumps [rxjava](https://github.com/ReactiveX/RxJava) from 2.2.5 to 2.2.18.
- [Release notes](https://github.com/ReactiveX/RxJava/releases)
- [Changelog](https://github.com/ReactiveX/RxJava/blob/v2.2.18/CHANGES.md)
- [Commits](ReactiveX/RxJava@v2.2.5...v2.2.18)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

agent/build.gradle

@@ -56,7 +56,7 @@ dependencies {
     compile 'net.bytebuddy:byte-buddy-agent:1.10.8'
 
     compileOnly 'io.projectreactor:reactor-core:3.2.5.RELEASE'
-    compileOnly 'io.reactivex.rxjava2:rxjava:2.2.5'
+    compileOnly 'io.reactivex.rxjava2:rxjava:2.2.18'
 
     jarFileTestCompile 'org.assertj:assertj-core:3.12.2'
     jarFileTestCompile 'junit:junit:4.12'

example/build.gradle

@@ -43,7 +43,7 @@ check.dependsOn(testReactor3_3_x)
 dependencies {
     testCompile project(path: ":agent", configuration: 'shadow')
     testCompile 'io.projectreactor:reactor-core:3.2.2.RELEASE'
-    testCompile 'io.reactivex.rxjava2:rxjava:2.2.5'
+    testCompile 'io.reactivex.rxjava2:rxjava:2.2.18'
 
     testCompile 'junit:junit:4.12'
     testCompile 'org.assertj:assertj-core:3.14.0'