modified gemspec to update rdoc

[skoba]

With recent gems, rdoc version 3.12 could not be installed, because it requires JSON ~> 1.4, that was not supported and could not build with its own library.
I updated rdoc version in gemspec to ~>6.1, and confirmed that happens no error with rspec.

[Stratus3D]

Thanks for the PR! Changes in the PR look good. But I see we only require Ruby 1.8.6 or greater whereas rdoc version 6.1 requires Ruby 2.4.0 or greater. I suppose this is only a development dependency, so maybe it doesn't matter.

[skoba]

Thanks for reply. In official, Ruby project supports only recent three versions, 2.4, 2.5, and 2.6, now, but I know 1.8.6 is active in somewhere still now. It could be controversy with HL7 version 2, which should be responsible with old Ruby.

[skoba]

Another idea would be separate version to support 1.8.6 -2.3.x from 2.4 or later. I would be wondering if ruby-hl7 version 1.4 could support rdoc 6.1 based on Ruby 2.4 or later, and keep 1.3.x to support rdoc 3.1.