Change rxjava version that is used in hystrix tests (#5335)

instrumentation/hystrix-1.4/javaagent/build.gradle.kts

@@ -17,7 +17,7 @@ dependencies {
   implementation(project(":instrumentation:rxjava:rxjava-1.0:library"))
 
   library("com.netflix.hystrix:hystrix-core:1.4.0")
-  library("io.reactivex:rxjava:1.0.7")
+  library("io.reactivex:rxjava:1.0.8")
 }
 
 tasks.withType<Test>().configureEach {