bump compileSdkVersion to 28 (#44)

Author: ingvardm

android/build.gradle

@@ -12,11 +12,11 @@ buildscript {
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 26
+    compileSdkVersion 28
 
     defaultConfig {
         minSdkVersion 16
-        targetSdkVersion 26
+        targetSdkVersion 28
         versionCode 1
         versionName "1.0"
     }