.travis.yml: change -std=c89 to -ansi

For C, -ansi is equivalent to -std=c90
For C++, -ansi is equivalent to -std=c++98

We also place -ansi in CPPFLAGS instead of the usual command line config,
to avoid getting it when linking (clang complains)

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8325)