travis: fix test_double_equal syntax-check failures.

* .travis/setup_lua.sh: Use portable "test x = y" expressions.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: revert removal of CI syntax checks.

* travis.yml.in (script): Reinstate accidentally removed CI
syntax checks.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>