which -s doesn't work on Debain

[Arcath]

I've found that Debain (and Ubuntu) dont support -s on which so the new rbenv code doesn't work so I get:

-----> Loading rbenv
       Illegal option -s
 !     rbenv not found
 !     If rbenv is installed, check your :rbenv_path setting.
 !     Command failed.
       Failed with status 1

If you remove -s on line 19 of mina/lib/rbenv.rb everything seems to work fine. I did a quick test on MacOS and Debian to make sure everything will work as expected:

MacOS:

$ if ! which rbenv >/dev/null; then; echo "not pathed right"; fi
$ if ! which rvm >/dev/null; then; echo "not pathed right"; fi
not pathed right

Debian:

$ if ! which rbenv >/dev/null; then echo "not pathed right"; fi
$ if ! which rvm >/dev/null; then echo "not pathed right"; fi
not pathed right

So it works as expected on both without the -s

Should be a nice quick fix, didn't really feel it was worth opening a pull request to remove 3 characters.

[Arcath]

Done a bit more testing and this fix is also needed for Fedora/Redhat and Arch

[rstacruz]

Seems I overlooked this, sorry--scheduled for v0.2.1.