Use Composer Runtime API to determine Drush version

[weitzman]

• Composer says it is safe to get own version
• When doing development on Drush itself, you might see see the version below which amuses me and seems "right enough" since the major version is correct.

Drush version : 12.9999999.9999999.9999999-dev

[greg-1-anderson]

Feels a bit odd, but it's documented, so I guess we can use it, and it's preferable to what we were doing before.

Could we preg_replace the 9s so that dev just comes out 12.x-dev? I understand why Composer does what it does, but I'd rather not show that as a version to the user. Unless you think folks will pass the output from drush --version version compare.

[weitzman]

Lets leave the version as is because it gives a sensible value for \Drush\Drush::getMinorVersion. Happy to discuss more.