Upgrade Kotlin (to 1.3), Coroutines, Gradle and Android gradle plugin.

Kotlin:                1.2.71 -> 1.3.50
Coroutines:            0.30.2 -> 1.3.1
Gradle:                4.6    -> 5.4.1
Android gradle plugin: 3.2.1  -> 3.5.0

This brings us down to *one* experimental coroutine API, and we've
opted in to using it in just *one* place.

(The fact that the API to opt-in to using an experimental API in a
specific place is *also* experimental surely will not come back to
bite us later.)

app/src/main/java/eu/kanade/tachiyomi/extension/util/ExtensionInstallReceiver.kt

@@ -7,10 +7,10 @@ import android.content.IntentFilter
 import eu.kanade.tachiyomi.extension.model.Extension
 import eu.kanade.tachiyomi.extension.model.LoadResult
 import eu.kanade.tachiyomi.util.launchNow
-import kotlinx.coroutines.experimental.CoroutineStart
-import kotlinx.coroutines.experimental.Dispatchers
-import kotlinx.coroutines.experimental.GlobalScope
-import kotlinx.coroutines.experimental.async
+import kotlinx.coroutines.CoroutineStart
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.async
 
 /**
  * Broadcast receiver that listens for the system's packages installed, updated or removed, and only

app/src/main/java/eu/kanade/tachiyomi/extension/util/ExtensionLoader.kt

@@ -13,8 +13,8 @@ import eu.kanade.tachiyomi.source.CatalogueSource
 import eu.kanade.tachiyomi.source.Source
 import eu.kanade.tachiyomi.source.SourceFactory
 import eu.kanade.tachiyomi.util.Hash
-import kotlinx.coroutines.experimental.async
-import kotlinx.coroutines.experimental.runBlocking
+import kotlinx.coroutines.async
+import kotlinx.coroutines.runBlocking
 import timber.log.Timber
 import uy.kohesive.injekt.Injekt
 import uy.kohesive.injekt.api.get