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
+  - os: linux
+    sudo: false
+  - os: osx
+    osx_image: xcode10.3
+    addons:
+      homebrew:
+        update: true
+        packages:
+          - flex
+          - gperf
+          - bison
 
 env:
   - MRUBY_CONFIG=travis_config.rb