Fix: #861

It's not incorrect to use scientific notation, but it tend
to throw people off a bit, so it's best to keep it for very large
numbers.

Avoid scientific notation before exponent 15

e.g.
```ruby
{ 1 => 2 }
```

The callback will be invoked for `1` as while it has a native JSON
equivalent, it's not legal as an object name.

`JSON::Coder` callback now recieve a second argument to mark object keys

The check for the modules dir here indicates whether we are on JDK
9+, since 1.8 did not have modules.

Fixes #862.

See #859 for some background.

[Java] implement a better fix for an out of bounds exception

Small JRuby build fixes