Fix indentation in koan/app.py

Making RPMs on Fedora 20 fails with:

    Bytecompiling .py files below .../cobbler/rpm-build/BUILDROOT/cobbler-2.5.0-1.fc20.x86_64/usr/lib/python2.7 using /usr/bin/python2.7
    Compiling .../cobbler/rpm-build/BUILDROOT/cobbler-2.5.0-1.fc20.x86_64/usr/lib/python2.7/site-packages/koan/app.py ...
    Sorry: IndentationError: unindent does not match any outer indentation level (app.py, line 693)
    error: Bad exit status from /var/tmp/rpm-tmp.OgToE3 (%install)
Merge pull request #775 from ciupicri/bugfix-indentation

Fix indentation in koan/app.py