Invoke rbenv alias <name> <version> to make a symbolic link from <name> to <version> in the rbenv versions directory, effectively creating an alias. The cool part is that if you pass in a point release as the name, you can give --auto to link to the latest installed patch level. For example, rbenv alias 1.8.7 --auto will automatically create an alias from 1.8.7 to 1.8.7-p371 (or whatever the most recent version you have installed is).

Plus, if you're using ruby-build, rbenv install A.B.C-pXXX automatically invokes rbenv alias A.B.C --auto, so you'll always have up to date aliases for point releases.

Whether it's a good idea to use these aliases in a .ruby-version file, I cannot say. I created this plugin to find out. If your only concern is having to reinstall gems every time you install a new patch release, check out rbenv-communal-gems.

mkdir -p "$(rbenv root)/plugins"
git clone https://github.com/tpope/rbenv-aliases.git \
  "$(rbenv root)/plugins/rbenv-aliases" &&\
rbenv alias --auto