Improve libsodium version API and check

This now exposes the libsodium version as:

  * RbNaCl::Sodium::Version::STRING
  * RbNaCl::Sodium::Version::MAJOR
  * RbNaCl::Sodium::Version::MINOR
  * RbNaCl::Sodium::Version::PATCH

The version check now compares the required version against the installed
version by first parsing them as integers and then using the spaceship operator.

The minimum version of libsodium has been changed to 0.4.3.