travis update attempt 5

`stretch` mxe binaries are failing due to missing libraries like
`libssl.so.1.1`, try to use the `trusty` repo instead.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>

installdeps

@@ -319,7 +319,7 @@ debian_installdeps() {
         # this is necessary to upgrade libcurl from trusty to xenial on travis
         sudo apt-get -qy -f install
 
-        debian_rel=stretch
+        debian_rel=trusty
 
         apt_ssl_exceptions=/etc/apt/apt.conf.d/80ssl-exceptions