Added Net-Ping Gem. Display Error Notice for Job Listing if No Internet

Please run 'bundle install' to install 'net-ping' gem.
Net-ping gem used to test if tcp connection possible to Google.com.
If unable to connect then socket error is triggered in jobs model to prevent error
trying perform api search at indeed URL.
If no internet connection then nothing is assigned to 'response' in jobs controller,
causing the 'else' condition to be realised. User is redirected to the 'users' index
page and a 'notice' is displayed in the menu _nav.html.erb notifying the user
that they need internet connection to display the job listing.