Add missing buildToolsVersion

.travis.yml

@@ -19,8 +19,7 @@ android:
     components:
         - tools
         - android-27
-        - build-tools-26.0.2
-        - build-tools-27.0.0
+        - build-tools-27.0.2
 
 install:
     - >

build.gradle

@@ -5,6 +5,7 @@ buildscript {
         kotlinVersion = '1.2.0'
         minSdkVersion = 19
         sdkVersion = 27
+        buildToolsVersion = '27.0.2'
         supportLibraryVersion = '27.0.2'
         takisoftFixVersion = '27.0.2.0'
         playServicesVersion = '11.6.2'

core/build.gradle

@@ -1,6 +1,7 @@
 apply plugin: 'com.android.library'
 
 android {
+    buildToolsVersion rootProject.buildToolsVersion
     compileSdkVersion rootProject.sdkVersion
 
     defaultConfig {

mobile/build.gradle

@@ -14,6 +14,7 @@ def getCurrentFlavor() {
 }
 
 android {
+    buildToolsVersion rootProject.buildToolsVersion
     compileSdkVersion rootProject.sdkVersion
     defaultConfig {
         applicationId "com.github.shadowsocks"

plugin/build.gradle

@@ -2,6 +2,7 @@ apply plugin: 'com.android.library'
 apply plugin: 'kotlin-android'
 
 android {
+    buildToolsVersion rootProject.buildToolsVersion
     compileSdkVersion rootProject.sdkVersion
 
     defaultConfig {