Fix so tests pass on travis-ci

I saw my other pull requests failed on travis, but this appears to be due to a [change](https://github.com/openresty/test-nginx/commit/b54ea7b3c4054eaf720de4885b01d00c91372882) in the newer version of `Test::Nginx` that now gets installed. The issue can be resolved by [upgrading valgrind to 3.8+](https://github.com/openresty/test-nginx/pull/17#issuecomment-51648594). This should make Travis happy again.