improved check for OpenSSL::KDF

[escoffon]

Some openssl distributions (e.g. the openssl library on macOS Mojave) don't export the OpenSSL::KDF symbol

[escoffon]

It occurred to me that it should be possible to implement hasKDF as a class method instead, so that the constant check is done just once, rather than for each new instance of KeyGenerator. I can investigate and push a different PR if you'd like me to.

[escoffon]

And this PR fixes #9

[ankane]

Hey @escoffon, thanks for another PR 👍 I think this can be simplified to

if defined?(OpenSSL::KDF.hkdf)
  # ....

[escoffon]

Ah yes of course. I really shouldn't write code late at night. I pushed a new commit on the PR. It's indirectly tested through my document tests.

[ankane]

Thanks!