[Install] Prevent empty SHELL failing installation

When running yadr via docker, $SHELL starts out empty/null, so that was
triggering a fatal method not found error during installation.

Rakefile

@@ -274,7 +274,7 @@ def install_prezto
   run %{ mkdir -p $HOME/.zsh.after }
   run %{ mkdir -p $HOME/.zsh.prompts }
 
-  if ENV["SHELL"].include? 'zsh' then
+  if "#{ENV['SHELL']}".include? 'zsh' then
     puts "Zsh is already configured as your shell of choice. Restart your session to load the new settings"
   else
     puts "Setting zsh as your default shell"