.travis: Print commit range for debugging TRAVIS_COMMIT_RANGE

Sometimes Travis gives test results for more commits than I expect it
to.  This should make it easy to figure out why, so we can distinguish
between "Travis is giving us a surprisingly large range" and
"validate.go has a buggy GitCommits".

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

Author: wking

.travis.yml

@@ -13,5 +13,6 @@ before_install:
 install: true
 
 script:
+  - echo ${TRAVIS_COMMIT_RANGE}
   - go run .tools/validate.go -range ${TRAVIS_COMMIT_RANGE}