Bump rxjava from 1.2.7 to 1.3.8

Bumps [rxjava](https://github.com/ReactiveX/RxJava) from 1.2.7 to 1.3.8.
- [Release notes](https://github.com/ReactiveX/RxJava/releases)
- [Changelog](https://github.com/ReactiveX/RxJava/blob/v1.3.8/CHANGES.md)
- [Commits](ReactiveX/RxJava@v1.2.7...v1.3.8)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

grails-datastore-gorm-rx/build.gradle

@@ -1,5 +1,5 @@
 dependencies {
-    api 'io.reactivex:rxjava:1.2.7'
+    api 'io.reactivex:rxjava:1.3.8'
     api project(":grails-datastore-gorm"), {
         exclude group:"org.grails", module:'grails-async'
         exclude group:"org.grails", module:'grails-datastore-core'