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

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 21bfa56..7da9440 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -21,6 +21,12 @@
 )
 
 maven_jar(
+    name = "commons_text",
+    artifact = "org.apache.commons:commons-text:1.2",
+    sha1 = "74acdec7237f576c4803fff0c1008ab8a3808b2b",
+)
+
+maven_jar(
     name = "gson",
     artifact = "com.google.code.gson:gson:2.8.2",
     sha1 = "3edcfe49d2c6053a70a2a47e4e1c2f94998a49cf",