Suppress ERB warning when running RSpec

[koic]

This PR bumps RSpec to 3.10 to suppress ERB warning when running RSpec.

% ruby -v
ruby 3.1.0dev (2021-09-20T13:01:58Z master 9770bf23b7) [x86_64-darwin19]
% cd path/to/letter_opener
% bundle update
% bundle exec rake
/Users/koic/.rbenv/versions/3.1.0-dev/lib/ruby/gems/3.1.0/gems/rspec-core-3.5.4/lib/rspec/core/configuration_options.rb:171:
warning: Passing safe_level with the 2nd argument of ERB.new is deprecated.
Do not use it, and specify other arguments as keyword arguments.
/Users/koic/.rbenv/versions/3.1.0-dev/lib/ruby/gems/3.1.0/gems/rspec-core-3.5.4/lib/rspec/core/configuration_options.rb:171:
warning: Passing trim_mode with the 3rd argument of ERB.new is deprecated.
Use keyword argument like ERB.new(str, trim_mode: ...) instead.

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64 examples, 0 failures

Randomized with seed 45327

[nashby]

@koic thanks!