.travis.yml: switch to newer osx image.

Default osx image runs Mac OS X 10.12, which apparently suffers from
infrequent socket failures affecting some tests. Later image runs
10.13...

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5986)

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index bd9d8f8..33c3751 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,8 @@
 dist: trusty
 sudo: required
 
+osx_image: xcode9.3
+
 language: c
 cache: ccache
 git: