travis: use Makefile target travis.codecov to install codecov

fix travis build error [1]::

  The command "local/py3/bin/pip install codecov" failed and exited with 127

Use the correct pip (python environment) from build environment::

  $(PY_ENV_BIN)/python -m pip

[1] https://travis-ci.org/github/asciimoo/searx/jobs/669701405#L590



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>