Make auto generate support rubies with semantic versioning

[gucki]

Running rbenv alias --auto should generate an alias for 2.1 which points to the latest 2.1 patchlevel/ version. Right now it only does:

1.8.7 => 1.8.7-p371
1.9.3 => 1.9.3-p429
2.0.0 => 2.0.0-p451

I think it'd be a good idea to have more aliases like this:

1.8.7 => 1.8.7-p371
1.9.3 => 1.9.3-p429
2.0.0 => 2.0.0-p451
2.1 => 2.1.5

Or alternatively:

1.8.7 => 1.8.7-p371
1.9.3 => 1.9.3-p429
2.0.0 => 2.0.0-p451
2.1.x => 2.1.5

However 2.1.x is not compatible with rvm, however 2.1 is.

[gucki]

BTW, the same should be done for jruby, so having a jruby-1.7 link to latest jruby-1.7.x. Probably the same for all other rubies which use semantic versioning.

[tpope]

I guess I would be okay with this, if you want to try a pull request.