Add .env to .gitignore

[RyanofWoods]

Summary

This file is useful for testing:

• solidus_paypal_braintree/spec/support/gateway_helpers.rb

  Lines 8 to 15 in 920a4e7

  	public_key: ENV.fetch('BRAINTREE_PUBLIC_KEY', 'dummy_public_key'),
  	private_key: ENV.fetch('BRAINTREE_PRIVATE_KEY', 'dummy_private_key'),
  	merchant_id: ENV.fetch('BRAINTREE_MERCHANT_ID', 'dummy_merchant_id'),
  	merchant_currency_map: {
  	'EUR' => 'stembolt_EUR'
  	},
  	paypal_payee_email_map: {
  	'EUR' => ENV.fetch('BRAINTREE_PAYPAL_PAYEE_EMAIL', 'paypal+europe@example.com')

It was removed from the file here:

• f9dc06d

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• I have written a thorough PR description.
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[elia]

@RyanofWoods I'm ok with merging this, but wondering if it can be set on your machine level .gitignore instead 🤔