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.
Add newline at sodium/version.rb EOF
Remove non-raising verification functions

Now all three (authenticators, signatures and boxes) will raise if there
are errors in verifying them.  This ensures that errors are dealt with,
or the program comes crashing to a halt.  Self-tests are also updated.