rbenv: Detect $RBENV_ROOT and act accordingly.

Same fix as with pyenv, if RBENV_ROOT is set, use that.

This also fixes issues for Homebrew users if they followed the caveat to
set RBENV_ROOT to use the homebrew created directories.

plugins/rbenv/rbenv.load

-if test -d $HOME/.rbenv/bin
-  set PATH $HOME/.rbenv/bin $PATH
-end
+if test -n "$RBENV_ROOT"
+  if test -d $RBENV_ROOT/bin
+    set PATH $RBENV_ROOT/bin $PATH
+  end
+
+  if test -d $RBENV_ROOT/shims
+    set PATH $RBENV_ROOT/shims $PATH
+  end
+else
+  if test -d $HOME/.rbenv/bin
+    set PATH $HOME/.rbenv/bin $PATH
+  end
 
-if test -d $HOME/.rbenv/shims
-  set PATH $HOME/.rbenv/shims $PATH
+  if test -d $HOME/.rbenv/shims
+    set PATH $HOME/.rbenv/shims $PATH
+  end
 end