Support RSA.import for all Ruby versions.

[rabajaj0509]

For Ruby versions < 2.4.0:

2.3.5 :006 > OpenSSL::PKey::RSA.new.respond_to?(:set_key)
 => false

This patch makes sure that RSA.import method is supported for all versions of Ruby.

[sourcelevel-bot]

Hello, @rahulbajaj0509! This is your first Pull Request that will be reviewed by SourceLevel, an automatic Code Review service. It will leave comments on this diff with potential issues and style violations found in the code as you push new commits. You can also see all the issues found on this Pull Request on its review page. Please check our documentation for more information.

[sourcelevel-bot]

SourceLevel has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).

You can see more details about this review at https://app.sourcelevel.io/github/jwt/ruby-jwt/pulls/333.

[excpt]

Fixes #332.