Add graphviz to Travis and Appveyor for testing (#173)

.travis.yml

   - linux
   - osx
 
+addons:
+  apt:
+    packages:
+      - graphviz
+      
 before_install:
-  - go get -u github.com/golang/lint/golint honnef.co/go/tools/cmd/...
+  - go get -u github.com/golang/lint/golint honnef.co/go/tools/cmd/...   
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update          ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; 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

appveyor.yml

 clone_folder: c:\go\src\github.com\google\pprof
 
+install:
+ - cinst graphviz.portable
+ 
 before_build:
  - go get github.com/ianlancetaylor/demangle
 
  - go build github.com/google/pprof
 
 test_script:
-  - go test ./...
+  - go test -v ./...

internal/driver/webui_test.go

 	}
 	for _, c := range testcases {
 		if c.needDot && !haveDot {
-			t.Log("skpping", c.path, "since dot (graphviz) does not seem to be installed")
+			t.Log("skipping", c.path, "since dot (graphviz) does not seem to be installed")
 			continue
 		}