Update .travis.yml (#435)

* bump Go versions;
* bump macOS / Xcode versions;
* use homebrew addon to install graphviz.

.travis.yml

 
 matrix:
   include:
-    - os: linux
-      go: 1.9.x
     - os: linux
       go: 1.10.x
+    - os: linux
+      go: 1.11.x
     - os: linux
       go: master
     - os: osx
-      osx_image: xcode6.4
-      go: 1.9.x
-      env: SKIP_GRAPHVIZ=1
-    - os: osx
-      osx_image: xcode6.4
+      osx_image: xcode8.3
       go: 1.10.x
-      env: SKIP_GRAPHVIZ=1
     - os: osx
-      osx_image: xcode6.4
-      go: master
-      env: SKIP_GRAPHVIZ=1
+      osx_image: xcode8.3
+      go: 1.11.x
     - os: osx
-      osx_image: xcode7.3
-      go: 1.9.x
+      osx_image: xcode8.3
+      go: master
     - os: osx
-      osx_image: xcode7.3
+      osx_image: xcode9.4
       go: 1.10.x
     - os: osx
-      osx_image: xcode7.3
-      go: master
+      osx_image: xcode9.4
+      go: 1.11.x
     - os: osx
-      osx_image: xcode8.3
-      go: 1.9.x
+      osx_image: xcode9.4
+      go: master
     - os: osx
-      osx_image: xcode8.3
+      osx_image: xcode10.1
       go: 1.10.x
     - os: osx
-      osx_image: xcode8.3
-      go: master
+      osx_image: xcode10.1
+      go: 1.11.x
     - os: osx
-      env: SKIP_BINUTILS=1
+      osx_image: xcode10.1
       go: master
 
 addons:
   apt:
     packages:
       - graphviz
-
+  homebrew:
+    packages:
+      - graphviz
+    update: true
+    
 before_install:
   - go get -u golang.org/x/lint/golint honnef.co/go/tools/cmd/...
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update ; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" && -z $SKIP_BINUTILS ]]; then brew install binutils ; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" && -z $SKIP_GRAPHVIZ ]]; then brew install graphviz; fi
 
 script:
   - gofmtdiff=$(gofmt -s -d .) && if [ -n "$gofmtdiff" ]; then printf 'gofmt -s found:\n%s\n' "$gofmtdiff" && exit 1; fi