Downgrade GnuTLS to avoid incompatibilities

The version of GnuTLS in an up-to-date Ubuntu 16.04 (3.4.10-4ubuntu1.8)
fails compat.sh. As a workaround until we fix this or decide that this
is a bug in the Ubuntu package, downgrade GnuTLS to the same upstream
version, but as originally released in Ubuntu 16.04, without the latest
Ubuntu packages.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/.travis.yml b/.travis.yml
index fc6a518..2d99621 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,7 +23,8 @@
 addons:
   apt:
     packages:
-    - gnutls-bin
+    - libgnutls30=3.4.10-4ubuntu1
+    - gnutls-bin=3.4.10-4ubuntu1
   coverity_scan:
     project:
       name: "ARMmbed/mbedtls"