Update Gradle version and targetSdkVersion #327

Google will control SDK versions starting August 2018.

Improving app security and performance on Google Play for years to come

August 2018: New apps required to target API level 26 (Android 8.0) or higher.
November 2018: Updates to existing apps required to target API level 26 or higher.
2019 onwards: Each year the targetSdkVersion requirement will advance. Within one year following each Android dessert release, new apps and app updates will need to target the corresponding API level or higher.