Add .env to .gitignore

This file is useful for testing:
- https://github.com/solidusio/solidus_paypal_braintree/blob/920a4e7da8e6b2cb441ca1400c8c3df0832aa01e/spec/support/gateway_helpers.rb#L8-L15

It was removed from the file here:
- f9dc06d

.gitignore

@@ -18,3 +18,4 @@ spec/examples.txt
 .rvmrc
 .ruby-version
 .ruby-gemset
+.env