Compare down cased segment names.

Mailchimp doesn't care about case when creating segments. It sees "Customers" as equal to "customers". Therefor we need to compare the passed in segment name in a case insensitive way.
Merge branch 'master' of https://github.com/DynamoMTL/spree_chimpy
Merge pull request #15 from johanb/master

Ignore case in segment name