.travis.yml: Don't require FETCH_HEAD

Master builds only have a 'git clone ...' [1] so FETCH_HEAD isn't
defined and git-validation crashes [2].  We don't want to be
hard-coding a range here, and should update git-validation to handle
these cases automatically.

Also echo TRAVIS_* variables during testing to make debugging
git-validation easier.

[1]: https://travis-ci.org/opencontainers/runc/jobs/213508696#L243
[2]: https://travis-ci.org/opencontainers/runc/jobs/213508696#L347

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

.travis.yml

@@ -23,8 +23,9 @@ before_install:
   - go get -u github.com/golang/lint/golint
   - go get -u github.com/vbatts/git-validation
   - go get -u github.com/mvdan/sh/cmd/shfmt
+  - env | grep TRAVIS_
 
 script:
-  - git-validation -run DCO,short-subject -v -range ${TRAVIS_BRANCH}..FETCH_HEAD
+  - git-validation -run DCO,short-subject -v
   - make BUILDTAGS="${BUILDTAGS}"
   - make BUILDTAGS="${BUILDTAGS}" clean validate test