Use variables to store some versions of core libs

build.gradle

@@ -1,10 +1,16 @@
 buildscript {
+  ext.versions = [
+          "kotlin"        : "1.3.0-rc-116",
+          "coroutinesCore": "0.27.0-eap13",
+          "retrofit"      : "2.4.0"
+  ]
+
   repositories {
     mavenCentral()
     maven { url 'http://dl.bintray.com/kotlin/kotlin-eap' }
   }
   dependencies {
-    classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.0-rc-57'
+    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${versions.kotlin}"
   }
 }
 
@@ -22,9 +28,9 @@ repositories {
 }
 
 dependencies {
-  api 'com.squareup.retrofit2:retrofit:2.4.0'
-  api 'org.jetbrains.kotlin:kotlin-stdlib:1.3.0-rc-57'
-  api 'org.jetbrains.kotlinx:kotlinx-coroutines-core:0.26.1-eap13'
+  api "com.squareup.retrofit2:retrofit:${versions.retrofit}"
+  api "org.jetbrains.kotlin:kotlin-stdlib:${versions.kotlin}"
+  api "org.jetbrains.kotlinx:kotlinx-coroutines-core:${versions.coroutinesCore}"
 
   testImplementation 'junit:junit:4.12'
   testImplementation 'com.squareup.okhttp3:mockwebserver:3.11.0'