Adjust gemspec to be able to install the gem under JRuby (bcrypt_pbkdf)

[dentarg]

From net-ssh/net-ssh#476 (comment)

On jruby there is no bcrypt_pbkdf but we don't require it on jruby. bcrypt_pbkdf is only required for reading password protected ed25519 keys, so if you have non encrypted keys or your keys are in agent then there is no need for that.

That is fine for me.

Not sure the test I added in #39 will continue to pass though... but I'm sure we can solve it somehow.

[dentarg]

Not sure the test I added in #39 will continue to pass though

$ bundle
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Using bundler 1.16.1
Using ed25519 1.2.4
Using net-ssh 5.0.2
Bundle complete! 2 Gemfile dependencies, 3 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ bundle exec ruby -e 'require "net/ssh"; p Net::SSH::Authentication::ED25519Loader::LOADED'
false

$ bundle exec ruby -e 'require "net/ssh"; require "ed25519"; p Net::SSH::Authentication::ED25519Loader::LOADED'
false

Seems like bcrypt_pbkdf is still needed.

$ bundle
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Using bcrypt_pbkdf 1.0.0
Using bundler 1.16.1
Using ed25519 1.2.4
Using net-ssh 5.0.2
Bundle complete! 3 Gemfile dependencies, 4 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ bundle exec ruby -e 'require "net/ssh"; p Net::SSH::Authentication::ED25519Loader::LOADED'
true

[dentarg]

bcrypt_pbkdf isn't needed when running JRuby – I think we can adjust the gemspec accordingly

$ ruby -v
jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 Java HotSpot(TM) 64-Bit Server VM 25.162-b12 on 1.8.0_162-b12 +jit [darwin-x86_64]

$ bundle
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Using bundler 1.16.5
Fetching ed25519 1.2.4 (java)
Installing ed25519 1.2.4 (java)
Fetching net-ssh 5.0.2
Installing net-ssh 5.0.2
Bundle complete! 2 Gemfile dependencies, 3 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ bundle exec ruby -e 'require "net/ssh"; p Net::SSH::Authentication::ED25519Loader::LOADED'
true