Fix user rbenv installs to use $HOME instead of ~

[cshaffer]

As a result of capistrano/sshkit#250 user installs of rbenv result in incorrect ENV in the mapped bins. SSHKit now quotes all ENV values, resulting in the rbenv root path to not be expanded, and thus invalid.

So what used to be this:
RBENV_ROOT=~/.rbenv RBENV_VERSION=2.2.2 ~/.rbenv/bin/rbenv exec gem query --quiet --installed --name-matches ^bundler$

Became this:
RBENV_ROOT="~/.rbenv" RBENV_VERSION="2.2.2" ~/.rbenv/bin/rbenv exec gem query --quiet --installed --name-matches ^bundler$

This is easy to workaround by explicitly setting the rbenv_path property, however the default behavior is broken when using the latest SSHKit.

This PR changes the install path to use $HOME instead of ~

[mattbrictson]

Related: capistrano/sshkit#303

[mattbrictson]

A fix is definitely needed, since capistrano-rbenv is now broken with sshkit 1.8.0. However I don't think this PR goes far enough. It requires developers to set a new :rbenv_user value, and even then, it makes assumptions about the location of that user's home directory being in /home (usually true, but not always).

[cshaffer]

Thanks for reviewing @mattbrictson, sorry for the hardcoding laziness before. Per your suggestion in capistrano/sshkit#303 I switched to use $HOME and verified this works locally as well.

[mattbrictson]

@cshaffer Great, thanks for verifying! It looks like we both had the same idea: I just opened #59 before seeing your commit. Oops!

@kirs Feel free to merge either one. Thanks

[kirs]

Thanks for your PR! I merged @mattbrictson's patch. Feel free to update to 2.0.4