Arch Linux

This is probably related to update of gcc to 8.1.0. Maybe a rebuild will fix it.

Steps to reproduce:

```
sudo pacman -Syu
python -c "import numpy"
```

Output:
```
Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/numpy/core/__init__.py", line 16, in <module>
from . import multiarray
ImportError: libgfortran.so.4: cannot open shared object file: No such file or directory

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "<string>", line 1, in <module>
File "/usr/lib/python3.6/site-packages/numpy/__init__.py", line 142, in <module>
from . import add_newdocs
File "/usr/lib/python3.6/site-packages/numpy/add_newdocs.py", line 13, in <module>
from numpy.lib import add_newdoc
File "/usr/lib/python3.6/site-packages/numpy/lib/__init__.py", line 8, in <module>
from .type_check import *
File "/usr/lib/python3.6/site-packages/numpy/lib/type_check.py", line 11, in <module>
import numpy.core.numeric as _nx
File "/usr/lib/python3.6/site-packages/numpy/core/__init__.py", line 26, in <module>
raise ImportError(msg)
ImportError:
Importing the multiarray numpy extension module failed. Most
likely you are trying to import a failed build of numpy.
If you're working with a numpy git repo, try `git clean -xdf` (removes all
files not under version control). Otherwise reinstall numpy.

Original error was: libgfortran.so.4: cannot open shared object file: No such file or directory
```