RuntimeError: can't modify frozen Array (Rollbar, Rails 5.1 upgrade)

Unable to use rspec and rollbar after upgrading to rails 5.

1. Create a Rails 4 app
2. Upgrade gemfile to use rails 5
3. Try adding rollbar gem/support

Standard config/environment.rb:

 # Load the Rails application.
require_relative 'application'

# Initialize the Rails application.
Rails.application.initialize!

Error when running rspec:

An error occurred while loading {path to specific spec file}
Failure/Error: require File.expand_path('../../config/environment', __FILE__)

RuntimeError:
  can't modify frozen Array
# ./config/environment.rb:6:in `<top (required)>'
# ./spec/rails_helper.rb:5:in `<top (required)>'
...
No examples found.

In most cases, that error is a red herring for something else.

When encountering it, don't get overwhelmed with the recurrent can't modify frozen Array error messages, and instead check the very first error that appears when running a spec.

For example:

Failure/Error: validate :uniqueness, if: 'should_be_unique?'

ArgumentError: Passing string to be evaluated in :if and :unless conditional options is not supported. Pass a symbol for an instance method, or a lambda, proc or block, instead.