Update change log

Author: k-stachowiak

ChangeLog

@@ -35,6 +35,8 @@ Changes
      produced by some optimizing compilers, showing up as failures in
      e.g. RSA or ECC signature operations. Reported in #1722, fix suggested
      by Aurelien Jarno and submitted by Jeffrey Martin.
+   * Re-enable certain ssl-opt.sh tests that depend on a more recent GnuTLS
+     version, as one has become available and integrated with the CI system.
 
 = mbed TLS 2.16.0 branch released 2018-12-21