Suppress ExpiredTargetSdkVersion lint error (temp)

The Play Store requires app to target sdk 26 at minimum. This is causing
a lint error that makes Travis fail. We're not ready to target sdk 26
yet (it requires some changes with the SYSTEM_OVERLAY window type), and
Red Moon's Play Store listing has some issues anyway, so it's ok to
suppress the error for now; it doesn't affect F-Droid.

app/build.gradle

@@ -72,7 +72,7 @@ android {
         main.java.srcDirs += 'src/main/kotlin'
     }
     lintOptions {
-        disable 'NewApi'
+        disable 'NewApi','ExpiredTargetSdkVersion'
     }
 }