Update version to 2.8.0

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 882dbb5..e0b016d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS 2.7.x branch released 2018-xx-xx
+= mbed TLS 2.8.0 branch released 2018-03-16
 
 Default behavior changes
    * The truncated HMAC extension now conforms to RFC 6066. This means