Set RBENV_ROOT

Otherwise rbenv init initializes with default location, i.e. gem
binaries cannot be found.

Author: trkoch

lib/mina/rbenv.rb

@@ -33,6 +33,7 @@
 task :'rbenv:load' do
   queue %{
     echo "-----> Loading rbenv"
+    #{echo_cmd %{export RBENV_ROOT="#{rbenv_path}"}}
     #{echo_cmd %{export PATH="#{rbenv_path}/bin:$PATH"}}
 
     if ! which rbenv >/dev/null; then