[master] 268bdfc Bump osx image

[Federico G. Schwindt]

commit 268bdfc01dee25cebb3b1330c6fea795c1f4a561
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Sat Apr 21 08:52:22 2018 +0100

    Bump osx image

diff --git a/.travis.yml b/.travis.yml
index 2ff305c..8a741f6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,7 +15,7 @@ matrix:
     compiler: clang
     env: CLANG=6.0 SAN_FLAGS="--enable-asan --enable-ubsan"
   - os: osx
-    osx_image: xcode9.2
+    osx_image: xcode9.3
     compiler: clang
   allow_failures:
   - os: osx