main: default travis commit range is unreliable

https://github.com/opencontainers/runc/pull/1378#issuecomment-287920287
points to an alternate approach.

Reported-by: Mrunal Patel <mrunalp@gmail.com>
Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

.travis.yml

@@ -10,11 +10,17 @@ before_install:
   - go get -u github.com/golang/lint/golint
   - mkdir -p $GOPATH/src/github.com/vbatts && ln -sf $(pwd) $GOPATH/src/github.com/vbatts/git-validation && go get ./...
 
+before_script:
+  - echo $TRAVIS_COMMIT
+  - echo $TRAVIS_BRANCH
+  - echo $TRAVIS_TAG
+  - echo $TRAVIS_BUILD_NUMBER
+  - echo $TRAVIS_REPO_SLUG
+
 install: true
 
 script:
   - go vet -x ./...
   - golint ./...
   - go build .
-  - go test -v ./...
-
+  - go test -v ./...

main.go

@@ -50,8 +50,8 @@ func main() {
 	var commitRange = *flCommitRange
 	if commitRange == "" {
 		if strings.ToLower(os.Getenv("TRAVIS")) == "true" && !*flNoTravis {
-			if os.Getenv("TRAVIS_COMMIT_RANGE") != "" {
-				commitRange = os.Getenv("TRAVIS_COMMIT_RANGE")
+			if os.Getenv("TRAVIS_BRANCH") != "" {
+				commitRange = fmt.Sprintf("%s..FETCH_HEAD", os.Getenv("TRAVIS_BRANCH"))
 			} else if os.Getenv("TRAVIS_COMMIT") != "" {
 				commitRange = os.Getenv("TRAVIS_COMMIT")
 			}