Update library version to 2.18.0

Increase the SO versions of libmbedx509 and libmbedtls due to the
addition of fields in publicly visible (non-opaque) structs:
  - mbedtls_ssl_config
  - mbedtls_ssl_context
  - mbedtls_x509_crt
diff --git a/ChangeLog b/ChangeLog
index bf835b6..6c7fce2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS x.x.x branch released xxxx-xx-xx
+= mbed TLS 2.18.0 branch released 2019-06-11
 
 Features
    * Add the Any Policy certificate policy oid, as defined in