altendky/pyqt-tools#38

* Add `bdist_wheel` to the uploaded distributions.  This avoids the need
  for pip to run `setup.py` when installing the package, speeding up
  installation and avoiding potential environment issues.
* `skip_existing` avoids CI failures due to "deploy" tasks being run
  several times by Travis for one tag pushed.

* Remove Python 3.4.
* Add Python 2 and 3.

Problems with the `warnings` module:

* Undesirable output in tests, although it could be filtered out.
* Output unsuited for users: shows source code of Python-dotenv.  Users
  don't know what to do with that.
* Rather meant for programming issues (e.g. deprecation) rather than
  runtime issues (file not found).

Problems with the `logging` module:

* Slightly less easy to test, unlike warnings with
  `catch_warnings(record=True)`, because you need to use the `mock`
  package for compatibility with Python 2.

Despite this last issue, I think it makes more sense to use the
`logging` module as a base for warnings.

Of course, `logging` is not suited to all forms of output, so this
change doesn't mean `logging` has to be used everywhere.  For instance,
if we want to improve the output of the `dotenv` CLI later, we might
want to use `print`, `click.echo` or other printing facilities.

Signed-off-by: Ulysses Souza <ulyssessouza@gmail.com>