use SDK 27, build tools 27.0.3

android/build.gradle

@@ -1,7 +1,7 @@
 apply plugin: 'com.android.library'
 
-def DEFAULT_COMPILE_SDK_VERSION             = 26
-def DEFAULT_BUILD_TOOLS_VERSION             = "26.0.3"
+def DEFAULT_COMPILE_SDK_VERSION             = 27
+def DEFAULT_BUILD_TOOLS_VERSION             = "27.0.3"
 def DEFAULT_MIN_SDK_VERSION                 = 16
 def DEFAULT_TARGET_SDK_VERSION              = 26