Test fails in directories other than ccd_utils

Test works in ccd_utils, but if we go upper directory, then we get problems.

(my-rdkit-env) [qyuan@ch-qyuan-z440 ccd_utils]$ nosetests test_pdb_chemical_components.py
....................................
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Ran 36 tests in 0.090s

OK
(my-rdkit-env) [qyuan@ch-qyuan-z440 ccd_utils]$ cd ..
(my-rdkit-env) [qyuan@ch-qyuan-z440 pdbe]$ nosetests ccd_utils/test_pdb_chemical_components.py

ERROR: Failure: ValueError (cannot read PDB chemical components from data/pdb_ccd_mmcif_test_files/HEM.cif as file not found)
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Traceback (most recent call last):
  File "/home/qyuan/anaconda2/envs/my-rdkit-env/lib/python2.7/site-packages/nose/loader.py", line 251, in generate
    for test in g():
  File "/home/qyuan/pdbe/ccd_utils/test_pdb_chemical_components.py", line 99, in test_load_hem_from_cif
    hem = PdbChemicalComponents(file_name=cif_filename('HEM'), cif_parser=cif_parser)
  File "/home/qyuan/pdbe/ccd_utils/pdb_chemical_components.py", line 80, in __init__
    self.read_ccd_from_cif_file(file_name)
  File "/home/qyuan/pdbe/ccd_utils/pdb_chemical_components.py", line 317, in read_ccd_from_cif_file
    raise ValueError('cannot read PDB chemical components from {} as file not found'.format(file_name))
ValueError: cannot read PDB chemical components from data/pdb_ccd_mmcif_test_files/HEM.cif as file not found