Tests: Don't fail when whoosh isn't installed

[hseg]

• papis version ($ papis --version or commit number):
  5a0799c

how to reproduce the issue

On a system without whoosh installed:

cd "$(mktemp -d)"
git clone https://github.com/papis/papis.git
cd papis/
python setup.py build
python -m pytest papis tests

Error log:
error.log

[hseg]

Problem found: test_cache_path is missing an importorskip("whoosh")

[Alex Fikl]

That sounds right to me! Want to make a pull request?

[hseg]

Oops, thought I had opened it