travis: install gnutls-bin package

This is required for gnutls-serv, which enables a few more tests.

Closes https://github.com/curl/curl/pull/2958
diff --git a/.travis.yml b/.travis.yml
index 5998f4c..202242f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -38,6 +38,7 @@
             - autopoint  # for libpsl that needs autoreconf that uses gettext that needs it
             - libunistring-dev # for libidn2 needed by libpsl
             - libnss3-dev
+            - gnutls-bin
 
 matrix:
     include: