Add config file for running tests with coverage report

To generate coverage report:
- install pytest-cov:
    ```
        pip install pytest-cov
    ```

- run `py.test` this way:
    ```
        py.test --cov PyDSTool --cov-report html --cov-config .coveragerc
    ```

- open in your browser the file `htmlcov/index.html`
Change command to run tests on Travis CI
Do not require 'matplotlib' installation
Remove 'distribute' auto installation
Make version check compatible with Python 2.6