Add pandoc to `install.sh` per build error

[hdoupe]

I got this error when building Tax-Brain:

______________________ ERROR collecting test_functions.py ______________________
ImportError while importing test module '/home/test_functions.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../test_functions.py:5: in <module>
    from compconfig import functions, helpers
/opt/conda/lib/python3.7/site-packages/compconfig/__init__.py:1: in <module>
    from .functions import *
/opt/conda/lib/python3.7/site-packages/compconfig/functions.py:7: in <module>
    from .constants import MetaParameters
/opt/conda/lib/python3.7/site-packages/compconfig/constants.py:4: in <module>
    from taxbrain import TaxBrain
/opt/conda/lib/python3.7/site-packages/taxbrain/__init__.py:3: in <module>
    from taxbrain.cli import *
/opt/conda/lib/python3.7/site-packages/taxbrain/cli.py:5: in <module>
    from taxbrain import TaxBrain, report
/opt/conda/lib/python3.7/site-packages/taxbrain/report.py:7: in <module>
    from .report_utils import (form_intro, form_baseline_intro, write_text, date,
/opt/conda/lib/python3.7/site-packages/taxbrain/report_utils.py:8: in <module>
    import pypandoc
E   ModuleNotFoundError: No module named 'pypandoc'

[hdoupe]

ping @andersonfrailey