~/.env: only use rbenv if it's installed

duh.

.env

@@ -32,7 +32,7 @@ add_to_path .bin .cabal .local/bin
export GEM_HOME=$HOME/.gem/ruby/2.1.0
export PATH=$HOME/.rbenv/bin:$GEM_HOME/bin:$PATH
eval "$(rbenv init -)"
which rbenv &> /dev/null && eval "$(rbenv init -)"
export M2_HOME=/opt/maven