Add ruby-build hook to pre-calculate g:ruby_version_paths.

[mtortonesi]

This hook script for ruby-build pre-calculates the value of the g:ruby_version_paths variable used by vim-ruby and vim-rbenv upon any interpreter installation or removal, thus leading to massive speedup times at Vim startup.

Further details and installation instructions are provided in the accompanying README.txt file.

[tpope]

Based on vim-ruby/vim-ruby#252, I assume your primary issue is that the existing static logic is failing for JRuby? Could we fix that?