Migration to new multiplatorm plugin (#947)

Migration to a new multiplatform plugin
  
  * kotlinx-coroutines-core-[common|js|native] are merged into one core module with multiple source sets
  * Folder structure and readme are restructured
  * Publication process is patched to preserve backward compatibility with artifact names
diff --git a/gradle.properties b/gradle.properties
index de04562..7d2270e 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -12,6 +12,7 @@
 dokka_version=0.9.16-rdev-2-mpp-hacks
 bintray_version=1.8.4-jetbrains-5
 byte_buddy_version=1.9.3
+reactor_vesion=3.2.5.RELEASE
 artifactory_plugin_version=4.7.3
 
 # JS