Update `.travis.yml` to support newer `bison` on MacOS.

If it doesn't work, I will revert the last update to `parse.y`.

.travis.yml

@@ -3,11 +3,17 @@ language: c
 sudo: false
 
 matrix:
-  include:
   - os: linux
     sudo: false
   - os: osx
-      osx_image: xcode7.1
+    osx_image: xcode10.3
+    addons:
+      homebrew:
+        update: true
+        packages:
+          - flex
+          - gperf
+          - bison
 
 env:
   - MRUBY_CONFIG=travis_config.rb