AndroidManifest.xml increase targetSdkVersion 19 -> 26

This is a Google Play store requirement as of November 1st, 2018.
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html

android/AndroidManifest.xml

@@ -48,7 +48,7 @@
             <!-- Background running -->
         </activity>
     </application>
-    <uses-sdk android:minSdkVersion="16" android:targetSdkVersion="19"/>
+    <uses-sdk android:minSdkVersion="16" android:targetSdkVersion="26"/>
 
     <!-- Needed to keep working while 'asleep' -->
     <uses-permission android:name="android.permission.WAKE_LOCK"/>