Added methods to `Platform` to check platform version

[basshelal]

• String getVersion() gets the system property "os.version" or null if it wasn't found
• int getVersionMajor() gets the "major" version number, that is the first number from getVersion() or -1 if none could be found
• int getVersionMinor() gets the "minor" version number, that is the second number from getVersion() or -1 if none could be found

Added a single unit test to verify all of the above.

This is regarding merging the hack from https://github.com/jnr/jnr-enxio/pull/39 to a more centralized location

Addresses but does not yet complete https://github.com/jnr/jnr-ffi/issues/260