Do not use make build on Travis

Travis is giving us a Python executable that it's already inside a virtualenv.
So, using that executable to create another virtualenv does not work properly
and all the packages are installed in the original virtualenv instead in the one
created by the Makefile; which makes packages to not be found at `venv/bin/`.

I'm using Travis `script` option to call the commands that we need just using
the virtualenv they provide to us instead of calling `make build`.

Author: humitos

.travis.yml

@@ -12,7 +12,8 @@ script:
   - powrap --check --quiet **/*.po
   - awk 1 dict dictionaries/*.txt > dict.txt
   - pospell -p dict.txt -l es_AR -l es_ES **/*.po
-  - make build
+  - pip install -q -r requirements.txt
+  - PYTHONWARNINGS=ignore::FutureWarning sphinx-build -j auto -W --keep-going -b html -d cpython/Doc/_build/doctree -D language=es . cpython/Doc/_build/html
 branches:
   only:
   - /^3\.\d$/