Migrate to python-etcd 0.4.3

Despite this release was very buggy it has really nice features:
* EtcdWatchTimedOut exception is raised when `watch` call timed out
* it supports SRV autodiscovery

Since we already implemented our own SRV discovery this feature is not
really interesting for us, but it solves the problem of having two
requirements files for different python versions, because python-etcd
will install dnspython or dnspython3 as a dependency.

In order to fix https://github.com/jplana/python-etcd/issues/152 and
https://github.com/jplana/python-etcd/pull/154 I had to override
`api_execute` method.

.travis.yml

@@ -4,8 +4,7 @@ python:
   - "3.4"
   - "3.5"
 install:
-  - if [[ $TRAVIS_PYTHON_VERSION == 2* ]]; then pip install -r requirements-py2.txt --use-mirrors; fi
-  - if [[ $TRAVIS_PYTHON_VERSION == 3* ]]; then pip install -r requirements-py3.txt; fi
+  - pip install -r requirements.txt
   - pip install coveralls codacy-coverage
 script:
   - python setup.py test