.travis.yml: Print TRAVIS_* environment variables

To make make debugging Travis environment issues more straightforward.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

.travis.yml

@@ -16,6 +16,7 @@ before_install:
 install: true
 
 script:
+  - env | grep TRAVIS_
   - make .govet
   - make .golint
   - echo "${TRAVIS_COMMIT_RANGE} -> ${TRAVIS_COMMIT_RANGE/.../..} (travis-ci/travis-ci#4596)"