fix version

build.gradle

@@ -26,7 +26,7 @@ dependencies {
     androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
         exclude group: 'com.android.support', module: 'support-annotations'
     })
-    compile 'com.android.support:appcompat-v7:25.3.1'
+    compile "com.android.support:appcompat-v7:$rootProject.ext.supportLibraryVersion"
     testCompile 'junit:junit:4.12'
     compile "org.jetbrains.kotlin:kotlin-stdlib:$rootProject.ext.kotlin_version"
 }