Enforce absolute RBENV_DIR
Enforce absolute RBENV_DIR

This prevents CDPATH from becoming an issue.

Closes #866.

nodenv/nodenv,nodenv/nodenv,jasonkarns/rbenv,jasonkarns/rbenv,sebhahn/pyenv,viclim/rbenv,yyyc514/rbenv,sstephenson/rbenv,pyenv/pyenv,sebhahn/pyenv,docwhat/rbenv,yyyc514/rbenv,sstephenson/rbenv,docwhat/rbenv,viclim/rbenv,yyuu/pyenv,pyenv/pyenv,yyyc514/rbenv,jrolfs/nodenv,jasonkarns/rbenv,yyuu/pyenv,docwhat/rbenv,rbenv/rbenv,OiNutter/nodenv,sstephenson/rbenv,cehoffman/rbenv,viclim/rbenv,cehoffman/rbenv,cehoffman/rbenv,OiNutter/nodenv,pyenv/pyenv,sebhahn/pyenv,rbenv/rbenv,jrolfs/nodenv