Fix BigDecimal support on Ruby 2.7

[diego-silva]

BigDecimal.new was deprecated on Ruby 2.6 and removed on Ruby 2.7. See more here.
The alternative is to use BigDecimal() which is supported on all rubies currently tested on this repo (2.4+).

This PR adds a reproducible test case for this BigDecimal issue, which fails on Ruby 2.7 before the proposed fix.

[codecov]

Codecov Report

Merging #50 (33013f1) into master (1fb3708) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   99.07%   99.09%   +0.01%     
==========================================
  Files           6        6              
  Lines         325      331       +6     
==========================================
+ Hits          322      328       +6     
  Misses          3        3              

Impacted Files	Coverage Δ
lib/sorbet-coerce/converter.rb	97.14% <100.00%> (+0.05%)	⬆️
spec/coerce_spec.rb	100.00% <100.00%> (ø)

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[donaldong]

Thanks! @diego-silva

[kyku]

TypeCoerce[BigDecimal].new.from(3.1415)
Traceback (most recent call last):
/home/kyku/.rbenv/versions/2.7.3/lib/ruby/gems/2.7.0/gems/sorbet-coerce-0.5.0/lib/sorbet-coerce/converter.rb:134:in `BigDecimal': can't omit precision for a Float. (ArgumentError)