Update Gradle for Kotlin 1.5, add space-dev repository (#2598)

* Update Gradle to 4.8.3, add space-dev repository
* Update atomicfu and binary compatibility validator
diff --git a/build.gradle b/build.gradle
index 171b16b..08d0778 100644
--- a/build.gradle
+++ b/build.gradle
@@ -65,6 +65,7 @@
         }
         maven { url "https://jetbrains.bintray.com/kotlin-native-dependencies" }
         maven { url "https://plugins.gradle.org/m2/" }
+        maven { url "https://maven.pkg.jetbrains.space/kotlin/p/kotlin/dev" }
     }
 
     dependencies {