Fix warnings and errors in cucumber feature tests (capistrano#2095)

* Fix failing apt-get due to unsupported Ubuntu ver

* Fix Gemfile warning due to missing source line

spec/support/Vagrantfile

@@ -6,7 +6,7 @@ Vagrant.configure("2") do |config|
   [:app].each_with_index do |role, i|
     config.vm.define(role, primary: true) do |primary|
       primary.vm.define role
-      primary.vm.box = "hashicorp/precise64"
+      primary.vm.box = "hashicorp/bionic64"
       primary.vm.network "forwarded_port", guest: 22, host: "222#{i}".to_i
       primary.vm.provision :shell, inline: "sudo apt-get -y install git-core"
 

spec/support/test_app.rb

@@ -40,6 +40,7 @@ def create_test_app
     FileUtils.mkdir(test_app_path)
 
     File.open(gemfile, "w+") do |file|
+      file.write "source 'https://rubygems.org'\n"
       file.write "gem 'capistrano', path: '#{path_to_cap}'"
     end