why is delta.py being skipped in my local doctests?

[Chris Smith]

Does anyone have an idea why the delta.py file in concrete is being skipped locally for me? It runs in travis, but not on my machine:

```

$ bin/doctest delta

============================= test process starts =============================

executable:         c:\Python26\python.exe  (2.6.6-final-0) [CPython]

architecture:       32-bit

cache:              yes

ground types:       python

hash randomization: off

sympy\functions\special\delta_functions.py[2] ..                           [OK]

sympy\integrals\deltafunctions.py[2] ..                                    [OK]

================== tests finished: 4 passed, in 0.34 seconds ==================

```

(concrete/delta.py is being missed)