Add RSpec setup with rspec-rails

Generated with the command:

rails generate rspec:install
Remove --warnings flag from .rspec file

This causes warnings to be printed about all of your dependencies, which
is untenable for a Rails project.

See http://stackoverflow.com/a/24177563.
Disable profiling of 10 slowest specs

This can be useful if you have a large, slow test suite, but until then
it's just noise.
Remove special treatment of specs tagged :focus

This adds noise to the output on every run, and we don't use it.
Merge pull request #2 from intercom/elk/rspec

Add RSpec setup with rspec-rails