Revert "travis: show git --version instead"

This reverts commit 7b5d57472bf93c90ae5761d0d7fb24de973758fa.

Git on Travis is 1.8.2. It is the cause of test failure on Travis!

    git version 1.8.2
Add "--force" to "git push --tags" to override existing tag

Git 1.8 requires "--force" option: