Stop invoking gosimple tool as it's deprecated. (#466)

Also stop supporting Go 1.10 as Go 1.12 is out.

.travis.yml

 
 matrix:
   include:
-    - os: linux
-      go: 1.10.x
     - os: linux
       go: 1.11.x
+    - os: linux
+      go: 1.12.x
     - os: linux
       go: master
     - os: osx
       osx_image: xcode8.3
-      go: 1.10.x
+      go: 1.11.x
     - os: osx
       osx_image: xcode8.3
-      go: 1.11.x
+      go: 1.12.x
     - os: osx
       osx_image: xcode8.3
       go: master
     - os: osx
       osx_image: xcode9.4
-      go: 1.10.x
+      go: 1.11.x
     - os: osx
       osx_image: xcode9.4
-      go: 1.11.x
+      go: 1.12.x
     - os: osx
       osx_image: xcode9.4
       go: master
     - os: osx
       osx_image: xcode10.1
-      go: 1.10.x
+      go: 1.11.x
     - os: osx
       osx_image: xcode10.1
-      go: 1.11.x
+      go: 1.12.x
     - os: osx
       osx_image: xcode10.1
       go: master
   - gofmtdiff=$(gofmt -s -d .) && if [ -n "$gofmtdiff" ]; then printf 'gofmt -s found:\n%s\n' "$gofmtdiff" && exit 1; fi
   - golintlint=$(golint ./...) && if [ -n "$golintlint" ]; then printf 'golint found:\n%s\n' "$golintlint" && exit 1; fi
   - go vet -all ./...
-  - gosimple ./...
   - ./test.sh
 
 after_success: