Add an option to repare incorrect record count

[bianjp]

Add an option repare_record_count to recalculate record count rather than using record count from header.

Useful when dbf file may be corrupted and record count in header is incorrect.

Example:

table = DBF::Table.new('data.dbf', nil, nil, repare_record_count: true)

Fix #79 .

@infused If you think this is ok, I'll add some tests later.

[bianjp]

The travis error relates to this travis-ci issue.