Moved osx_image

Set osx_image under each configuration, as it decides the OS (and
kernel) version not only Xcode version, and the configuration name
contains the kernel version.

.travis.yml

@@ -21,8 +21,6 @@ language: c
 
 dist: xenial
 
-osx_image: xcode11
-
 git:
   quiet: true
 
@@ -334,6 +332,7 @@ env:
 
   - &x86_64-darwin18
     name: x86_64-darwin18
+    osx_image: xcode11
     <<: *osx
     env:
       - CONFIG_FLAG=--with-opt-dir=/usr/local/opt/openssl@1.1:/usr/local/opt/zlib