`BigDecimal.new` has been deprecated in BigDecimal 1.3.3
which will be a default for Ruby 2.5.
Refer ruby/bigdecimal@5337373
* This commit has been made as follows:
```ruby
$ cd activemodel/
$ git grep -l BigDecimal.new | grep \.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```
* This commit has been tested with these Ruby versions:
```
ruby 2.5.0dev (2017-12-15 trunk 61262) [x86_64-linux]
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux]
ruby 2.3.5p376 (2017-09-14 revision 59905) [x86_64-linux]
ruby 2.2.8p477 (2017-09-14 revision 59906) [x86_64-linux]
```
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