`pyimport_conda` doesn't seem to handle superseded packages correction.

Recently the main conda channel added keras-2.2.4, but some of our internal legacy code requires 1.2.2. As a result, we've been getting this superseded notice:

```
The following packages will be SUPERSEDED by a higher-priority channel:

keras pkgs/main::keras-2.2.4-0 --> conda-forge::keras-1.2.2-py27_0
```

Normally this isn't an issue, but it `pyimport_conda` doesn't seem to finding the downgraded version:

```
)
ImportError('cannot import name activity_l1l2',)
File "/root/.julia/v0.6/Conda/deps/usr/lib/python2.7/site-packages/private-package/__init__.py", line 15, in
from private-package.main import run
File "/root/.julia/v0.6/Conda/deps/usr/lib/python2.7/site-packages/private-package/main.py", line 7, in
...
from keras.regularizers import activity_l1l2 # l1l2 for weight regularization
```

It seems that simply re-loading the package that's using `pyimport_conda` in the `__init__` resolves the issue.

Start with the pyimport_conda entry point and reproduce the keras 1.2.2 versus 2.2.4 superseded-package case described in the report. Compare the initial import with the behavior after reloading the package that uses pyimport_conda. Done means the downgraded keras package can be imported without requiring a manual reload.

Rédigé par le modèle d'indexation à partir du texte de l'issue.