Added CI_SERVER* environment variables to build process.

The environment variable CI_SERVER can be used by the build process to
switch some specific CI configuration, like this for example:
```ruby
require 'rspec/retry'
RSpec.configure do |config|
  config.default_retry_count = 3 if ENV['CI_SERVER']
end
```
The CI_SERVER_* variables were added for completeness.
Name should not be configurable. It's brand name!
Merge pull request #117 from cabeca/environment-variables

Added CI_SERVER* environment variables to build process.