DRY: Only define dependencies in install_requires

[hugovk]

• Fixes Add requirements.txt file for developpers #299.
• Alternative to Add requirements.txt #300.

Instead of defining the installation dependencies in .travis.yml, tox.ini and setup.py, only define the in setup.py. requirements.txt is not needed.

tox.ini does not need to duplicate them: it will run python setup.py install anyway, and install them from setup.py's install_requires.

Let's also use tox on Travis CI, no need to list them there either.

[pietermarsman]

Looking good.

How would you set up the dev environment? It would be nice if we could use a command that only installs the dependencies of the setup.py file, but not pdfminer.six itself.

[pietermarsman]

There are also depencies for development: tox and nose. These can be added using extra_requires.

How would you install the packages for development? The best way I can find is pip install -e .[dev] (assuming there is an extra_requires section called "dev"). It installs the package in editable mode allowing to update the source without reinstalling. And it installs the development packages.

And a last request for this PR is to add some documentation. I think the CONTRIBUTING.md file should have a small section on setting up a development environment and the commands for cloning the repo and installing the dev dependencies.

[hugovk]

Done and done!