travis: fix codecov

travis: fix codecov

In e017f69376da ("travis: run pep8/flake8 tests"), codecov
was removed from the install step, on the basis that tox-travis
would pull it in automatically.

This, it turns out, isn't entirely true: it is pulled in to the
tox environment, but the data is actually sent in an after_success
step. That is outside the tox environment, as it should be - if it
were part of the tox environment, running `tox -e coverage` on a
developer laptop would try to send data to the web. But, as codecov
now isn't present outside the tox environment, we see in the logs:

$ codecov
codecov: command not found

We don't get any reporting of success/failure in the after_success
step, so we didn't notice.

Restore the installation in the travis environment.

Fixes: e017f69376da ("travis: run pep8/flake8 tests")
Signed-off-by: Daniel Axtens <dja@axtens.net>