fix: install aws_ec2_environment from RubyGems rather than GitHub (#631)

I swear I'd already changed this but apparently not - we published this
gem to RubyGems ages ago, so there's no need to have new apps pull it
from GitHub

Author: G-Rath

variants/deploy_with_ackama_ec2_capistrano/template.rb

@@ -16,7 +16,7 @@
     gem "capistrano-rbenv", require: false
     gem "ed25519", require: false
     gem "bcrypt_pbkdf", require: false
-    gem "aws_ec2_environment", github: "ackama/aws_ec2_environment"
+    gem "aws_ec2_environment", require: false
 
     # capistrano-locally is required to install the app on to a freshly
     # provisioned EC2 instance