use OpenSSL::Digest

Digest::Digest is deprecated, so use Digest instead
Merge pull request #51 from AlexeyDemidov/fix/openssl_digest_digest_deprecation

use OpenSSL::Digest
Add gem version badge to readme
Bump version to v1.9.0
Make depedency specifier less strict
Add license to gemspec
Update link to docs
Support charge_customer_at
Remove any nil values before sending params to GC
Merge pull request #53 from gocardless/charge-customer-at

Support charge_customer_at
Merge branch 'master' into pagination