buildscripts,travis: fetch from mvn with retries (#4140)

A band aid for #3284, to make its symptoms less noticeable. 
diff --git a/build.gradle b/build.gradle
index f285689..7e5d009 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,6 +1,5 @@
 buildscript {
   repositories {
-    mavenCentral()
     mavenLocal()
     maven {
       url "https://plugins.gradle.org/m2/"
@@ -59,11 +58,11 @@
     targetCompatibility = 1.6
 
     repositories {
-        mavenCentral()
-        mavenLocal()
         maven {
-          url "https://oss.sonatype.org/content/repositories/snapshots/"
+            // The google mirror is less flaky than mavenCentral()
+            url "https://maven-central.storage-download.googleapis.com/repos/central/data/"
         }
+        mavenLocal()
     }
 
     [compileJava, compileTestJava, compileJmhJava].each() {