Prepare tools/r8_release.py for branch 3.1

Change-Id: I1e1581cb3ce29dd0fa650c6fe3275bc4ac4e9d70

tools/r8_release.py

diff --git a/tools/r8_release.py b/tools/r8_release.py
index 6cee2f8..7914bf9 100755
--- a/tools/r8_release.py
+++ b/tools/r8_release.py
@@ -15,7 +15,7 @@
 
 import utils
 
-R8_DEV_BRANCH = '3.0'
+R8_DEV_BRANCH = '3.1'
 R8_VERSION_FILE = os.path.join(
     'src', 'main', 'java', 'com', 'android', 'tools', 'r8', 'Version.java')
 THIS_FILE_RELATIVE = os.path.join('tools', 'r8_release.py')