Update Go modules

[smaeda-ks]

Just trying to update modules...

I'm updating tests in custom_tls_certificate_test as I immediately noticed that their fixtures are pretty old and the response data format is different than what we return today, and it was causing errors after updating the jsonapi module:

$ make all
==> Downloading Go module
==> Downloading development dependencies
==> Tidying module
==> Running gofmt
==> Fixing imports
==> Testing go-fastly
--- FAIL: TestClient_ListCustomTLSCertificates_validation (0.00s)
    custom_tls_certificate_test.go:145: Trying to Unmarshal an object of type "", but "tls_certificate" does not match
--- FAIL: TestClient_CustomTLSCertificate (0.00s)
    custom_tls_certificate_test.go:40: Trying to Unmarshal an object of type "", but "tls_certificate" does not match

b71bd99#diff-c0c259c2589ed26c6456d3f1a503dfad3bbf0d45a28b5ff913b90a86a687df6cL15-R15

you notice that the old fixture had an extra [{"data":{ block.

[smaeda-ks]

cc: @Integralist