Update maven central url

The central repository no longer support insecure communication.
see [1] for more details.

[1]: https://support.sonatype.com/hc/en-us/articles/360041287334

Change-Id: I5386e400df48435bcc92a2c14a25032e4d77174f
diff --git a/tools/util.py b/tools/util.py
index fb49664..2c731e6 100644
--- a/tools/util.py
+++ b/tools/util.py
@@ -27,7 +27,7 @@
   'GERRIT': 'http://gerrit-maven.storage.googleapis.com',
   'GERRIT_API': 'https://gerrit-api.commondatastorage.googleapis.com/release',
   'ECLIPSE': 'https://repo.eclipse.org/content/groups/releases',
-  'MAVEN_CENTRAL': 'http://repo1.maven.org/maven2',
+  'MAVEN_CENTRAL': 'https://repo1.maven.org/maven2',
   'MAVEN_LOCAL': 'file://' + path.expanduser('~/.m2/repository'),
 }