Allows setting project URL for tests.

Using NEMEX_URL environment variable, you can set up a different URL for
the project to be used in tests on different environments. If none is
set, the default one on http://*.xip.io/ (provided by Vagrant) is used.
Add travis build configuration.

Installs apache and PHP-FPM, using the virtual host configuration
existing on /build/travis.

Adds target `selenium-kill` and prevents `tests-functional` from
ignoring errors, which otherwise would make the build never fail.
Add Travis badge on README.
Merge pull request #9 from augustohp/6-travis

Enable Travis testing.