Upgrade java-prettify to 1.2.2 and consume from Maven Central

The latest version available is actually 2.0.3, but upgrade to
1.2.2 because that's the closest version available to the version
currently being used (1.2.1) and is less likely to cause problems.

Change-Id: Ib8c06dffb1a2bf2b7efc1499fd1d6613e951e16f

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 7fd84b6..ddf8621 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -47,9 +47,8 @@
 
 maven_jar(
     name = "prettify",
-    artifact = "prettify:java-prettify:1.2.1",
-    repository = GERRIT,
-    sha1 = "29ad8d072f9d0b83d1a2e9aa6ccb0905e6d543c6",
+    artifact = "com.github.twalcari:java-prettify:1.2.2",
+    sha1 = "b8ba1c1eb8b2e45cfd465d01218c6060e887572e",
 )
 
 COMMONMARK_VERSION = "0.9.0"