Update the version of the library to 2.1.18
diff --git a/ChangeLog b/ChangeLog
index 1d73308..8b9b8c3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS 2.1.xx branch released xxxx-xx-xx
+= mbed TLS 2.1.18 branch released 2018-12-21
 
 Bugfix
    * Fix for Clang, which was reporting a warning for the bignum.c inline