[Fix] Startpage ValueError on Spanish date format

[mod] requirements.txt: upgrade dependency versions.

[add] Additional Wikimedia projects

[add] rubygems engine

[mod] .travis.yml: add Python 3.7 and Python 3.8

Suggested by @Venca24 <https://github.com/asciimoo/searx/pull/1914#issuecomment-610194209>.

A list of the added projects:
- Wikibooks
- Wikinews
- Wikiquote
- Wikisource
- Wiktionary
- Wikiversity
- Wikivoyage

improve travis build

BTW: separate env output in the travis log
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

[fix] travis: fix docker tag.

the "-dirty" ignores the searx/brand.py and utils/brand.env files.

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

revert "makefile.python: ... drop build support py2" to get back Py2 support.
TPy2 support need virtualenv installed by the OS.

BTW: log environment and python version in travis's install phase
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

- replace: $(PY_ENV_BIN)/pylint --> $(PY_ENV_BIN)/python -m pylint
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

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#L590Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

[1] https://github.com/asciimoo/searx/pull/1910#issuecomment-607244871Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

This reverts commit eb87b07d.

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

Fix "Permission denied: '/home/travis/.cache/" errors from travis build [1].

Change owner https://docs.travis-ci.com/user/caching/#caches-and-read-permissions
and remove old log in the *before_caching* phase:

  https://docs.travis-ci.com/user/caching/#before_cache-phase

About pip cache see: https://docs.travis-ci.com/user/caching/#pip-cacheSigned-off-by: Markus Heiser <markus.heiser@darmarit.de>

[1] https://travis-ci.org/github/asciimoo/searx/jobs/669664131#L220

From py3.3 on a subset of virtualenv is built-in, so you can run '-m venv' ot of
the box.

- replace: $(PY_ENV_BIN)/pip --> $(PY_ENV_BIN)/python -m pip
- remove obsolete virtualenv-exe target and adjust VTENV_OPTS
- remove obsolete msg-pip-exe target
- print list of py launchers available from $(PY_ENV_BIN) to the log
- fix hard coded ./local
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

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

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

[fix] travis: fix docker build.

[fix] template: fix typo in about.html

export build variables / support brands