Update rxjava [2.1.16 -> 2.2.7]

Author: greenrobot-team

objectbox-rxjava/build.gradle

@@ -7,7 +7,7 @@ sourceCompatibility = 1.7
 
 dependencies {
     compile project(':objectbox-java')
-    compile 'io.reactivex.rxjava2:rxjava:2.1.16'
+    compile 'io.reactivex.rxjava2:rxjava:2.2.7'
 
     testCompile 'junit:junit:4.12'
     testCompile 'org.mockito:mockito-core:2.25.1'