Add warning about upgrading commonmark

When commonmark is updated it should also be updated in the
gitiles plugin project.

Also add a constant for the version to reduce amount of places
it needs to be modifed.

Change-Id: Icb3fdea76c4d7a87e83806a93b7ba43b83485ee6

lib/BUCK

diff --git a/lib/BUCK b/lib/BUCK
index 9e64d07..fadcefd 100644
--- a/lib/BUCK
+++ b/lib/BUCK
@@ -40,16 +40,18 @@
   repository = GERRIT,
 )
 
+COMMONMARK_VERSION = '0.6.0'
+# When upgrading commonmark it should also be updated in plugins/gitiles
 maven_jar(
   name = 'commonmark',
-  id = 'com.atlassian.commonmark:commonmark:0.6.0',
+  id = 'com.atlassian.commonmark:commonmark:' + COMMONMARK_VERSION,
   sha1 = '5df3f6fa3073966620685924aa22d08ece7213f2',
   license = 'commonmark',
 )
 
 maven_jar(
   name = 'cm-autolink',
-  id = 'com.atlassian.commonmark:commonmark-ext-autolink:0.6.0',
+  id = 'com.atlassian.commonmark:commonmark-ext-autolink:' + COMMONMARK_VERSION,
   sha1 = '4d7e828a4651e2f590b4a059925991be58e62da6',
   license = 'commonmark',
   deps = [
@@ -67,7 +69,7 @@
 
 maven_jar(
   name = 'gfm-strikethrough',
-  id = 'com.atlassian.commonmark:commonmark-ext-gfm-strikethrough:0.6.0',
+  id = 'com.atlassian.commonmark:commonmark-ext-gfm-strikethrough:' + COMMONMARK_VERSION,
   sha1 = '75a95aaec77810496de41239bcc773adfb13285f',
   license = 'commonmark',
   deps = [':commonmark'],
@@ -75,7 +77,7 @@
 
 maven_jar(
   name = 'gfm-tables',
-  id = 'com.atlassian.commonmark:commonmark-ext-gfm-tables:0.6.0',
+  id = 'com.atlassian.commonmark:commonmark-ext-gfm-tables:' + COMMONMARK_VERSION,
   sha1 = 'ae1c701517e8116bc205b561b9b215a53df8abc7',
   license = 'commonmark',
   deps = [':commonmark'],