pytest 3 breaks `test_ssh.py` test module

Running with `pytest=3.0.3` (which is currently installed by default in the travis build) I get the following

```
$ pytest odo/backends/tests/test_ssh.py
======================================================== test session starts =========================================================
platform darwin -- Python 2.7.12, pytest-3.0.3, py-1.4.31, pluggy-0.4.0
rootdir: /Users/rqa267/blaze/odo, inifile:
collected 0 items / 1 errors

=============================================================== ERRORS ===============================================================
__________________________________________ ERROR collecting odo/backends/tests/test_ssh.py ___________________________________________
Using @pytest.skip outside of a test (e.g. as a test function decorator) is not allowed. Use @pytest.mark.skip or @pytest.mark.skipif instead.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
```

This is caused by the recent release of pytest 3, which breaks the use of `pytest.skip` imperatively outside of a test function, as discussed in pytest-dev/pytest#1959

This can be fixed in the build by installing `'pytest<3'`, in `.travis.yml`, but I would need to go through an approval process to submit a new PR, so I'll leave it to some other intrepid soul.