Merge pull request #64 from OneSignal/update-compilesdkv

updated compileSdkVersion from 27 to 28

Author: jkasten2

android/build.gradle

@@ -22,7 +22,7 @@ rootProject.allprojects {
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 27
+    compileSdkVersion 28
 
     defaultConfig {
         minSdkVersion 16
@@ -72,4 +72,4 @@ rootProject.childProjects.each { projName, proj ->
     proj.afterEvaluate {
         DefaultManifestPlaceHolders.addManifestToAppProject(proj)
     }
-}
+}