build: update gradle version

build.gradle

@@ -54,7 +54,7 @@ buildscript {
         google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.1.2'
+        classpath 'com.android.tools.build:gradle:3.1.3'
         classpath rootProject.ext.deps["kotlin-gradle-plugin"]
 
         // NOTE: Do not place your application dependencies here; they belong

chrome-tabs/src/main/java/de/mrapp/android/tabswitcher/layout/AbstractTabViewHolder.java

@@ -37,8 +37,4 @@ public abstract class AbstractTabViewHolder {
      */
     public ImageButton closeButton;
 
-    /**
-     * The custom button of a tab.
-     */
-    public ImageButton customButton;
 }