Populate custom definitions before executing code for `--list`

One bug with `rbenv install` and custom definition paths (https://github.com/sstephenson/ruby-build/pull/614, https://github.com/sstephenson/ruby-build/pull/613) is its use with the `--list` switch.

The code that I have moved in this mock PR ensures that the `RUBY_BUILD_DEFINITIONS` list of paths is properly populated before calling the `definitions` function in `--list`.

I'm sure there's a more style-conformant solution, but this gets the job done.

/cc @mislav