* Bump Rubocop to 0.57.2
* Style/StderrPuts: Use warn instead of .puts
* Style/ExpandPathArguments: Use expand_path('../test_helper', __dir__) instead of expand_path('../../test_helper', __FILE__)
* Style/Encoding: Unnecessary utf-8 encoding comment
* Style/StringLiterals: Prefer double-quoted strings
* Style/AccessModifierDeclarations
* Style/FormatStringToken: Prefer annotated tokens
* Naming/UncommunicativeMethodParamName
* Metrics/LineLength: set maximum line length to 100 characters
* Style/IfUnlessModifier: Favor modifier if usage when having a single-line body
* Style/ClassVars
* Metrics/LineLength: set maximum line length to 80 characters (default)
* Style/AccessModifierDeclarations: EnforcedStyle: inline
Adds the magic `frozen_string_literal: true` comment to every file and
enables a Rubocop rule to make sure that it's always going to be there
going forward as well.
See here for more background [1], but the basic idea is that unlike many
other languages, static strings in code are mutable by default. This has
since been acknowledged as not a particularly good idea, and the
intention is to rectify the mistake when Ruby 3 comes out, where all
string literals will be frozen. The `frozen_string_literal` magic
comment was introduced in Ruby 2.3 as a way of easing the transition,
and allows libraries and projects to freeze their literals in advance.
I don't think this is breaking in any way: it's possible that users
might've been pulling out one of are literals somehow and mutating it,
but that would probably not have been useful for anything and would
certainly not be recommended, so I'm quite comfortable pushing this
change through as a minor version.
As discussed in #641.
[1] https://stackoverflow.com/a/37799399
Redefining the constant like this produces a warning:
```
$ bundle exec rake
/Users/brandur/stripe/stripe-ruby/test/stripe/ephemeral_key_test.rb:75: warning: already initialized constant Stripe::EphemeralKeyTest::FIXTURE
/Users/brandur/stripe/stripe-ruby/test/stripe/ephemeral_key_test.rb:6: warning: previous definition of FIXTURE was here
Loaded suite /Users/brandur/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rake-11.1.2/lib/rake/rake_test_loader
Started
...
```
They also don't appear to be used, so it should be fine just to strip
them out of the test suite.
