only download from jCenter as a last resort

"Artifact integrity verification aside, not only should jcenter() always be
last, but you need to put mavenCentral() before them. JCenter and Bintray
keep proving they're not a trustworthy artifact host. Ideally you fetch
nothing from them."
https://twitter.com/JakeWharton/status/1073102730443526144

https://blog.autsoft.hu/a-confusing-dependency/

build.gradle

@@ -1,7 +1,8 @@
 buildscript {
     repositories {
+        mavenCentral()
         maven { url 'https://maven.google.com/' } // :-| must be before jcenter()
-        jcenter()
+        jcenter() // download from jCenter as last resort https://blog.autsoft.hu/a-confusing-dependency
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:3.1.1'
@@ -9,7 +10,8 @@ buildscript {
 }
 allprojects {
     repositories {
+        mavenCentral()
         maven { url 'https://maven.google.com/' } // :-| must be before jcenter()
-        jcenter()
+        jcenter() // download from jCenter as last resort https://blog.autsoft.hu/a-confusing-dependency
     }
 }