[dump_formular] lower minimum bv value

[MaKru11]

Hi,

please consider lowering the minimum asserted value for bv in the dump_formular function, as it can in some cases be lower. https://github.com/DissectMalware/xlrd2/blob/74cd5302f9007f6cf9389f8819cf1676a1e2a717/xlrd2/formula.py#L2451

Example: sha256: 1df68d55968bb9d2db4d0d18155188a03a442850ff543c8595166ac6987df820 with minimum bv=70

There are 2 open issues on https://github.com/DissectMalware/XLMMacroDeobfuscator considering this file and issue: https://github.com/DissectMalware/XLMMacroDeobfuscator/issues/117 https://github.com/DissectMalware/XLMMacroDeobfuscator/issues/116