Stop using deprecated StringEscapeUtils from commons-lang3

Since version 3.6 of lang3, StringEscapeUtils is deprecated in favor
of the class of the same name in commons-text.

Add a dependency on commons-text, and use StringEscapeUtils from there
instead.

Keep the dependency on commons-lang3 since there is still a dependency
on it (TocFormatter uses StringUtils).

Change-Id: I99448c811672c9ac293e88acbeb99d566a8aebe7

lib/BUILD

diff --git a/lib/BUILD b/lib/BUILD
index 917f5f0..9ee64d5 100644
--- a/lib/BUILD
+++ b/lib/BUILD
@@ -9,6 +9,7 @@
     "autolink",
     "commonmark",
     "commons-lang3",
+    "commons-text",
     "cm-autolink",
     "gfm-strikethrough",
     "gfm-tables",