Upgrade gem to ruby 2.6

[ck2000]

I am receiving some deprecating warnings with ruby 2.6.3 e.g.:

warning: BigDecimal.new is deprecated; use BigDecimal() method instead.

If I am running via rails console the following:

tax = Taxger::Lohnsteuer.calculate(2015, stkl: 1, lzz: 1, re4: 70_000 * 100)

Traceback (most recent call last):
        1: from (irb):5
NoMethodError (undefined method `divide' for 0.0:BigDecimal)
Did you mean?  divmod

Could someone please help me out with these two messages?
Cheers

[ck2000]

I am been trying the solve the deprecated warnings by replacing BigDecimal.new is deprecated; use BigDecimal(). Now I am receiving another error message:

2.6.3 :001 > tax = Taxger::Lohnsteuer.calculate(2015, stkl: 1, lzz: 1, re4: 70_000 * 100)
"year {:stkl=>1, :lzz=>1, :re4=>7000000}"
Traceback (most recent call last):
        5: from (irb):1
        4: from lib/taxger/lohnsteuer.rb:24:in `calculate'
        3: from lib/taxger/lohnsteuer.rb:24:in `map'
        2: from lib/taxger/lohnsteuer.rb:24:in `each'
        1: from lib/taxger/lohnsteuer.rb:25:in `block in calculate'
NoMethodError (private method `BigDecimal' called for Taxger::Lohnsteuer:Module)