Update Rake >= 12.3.3

Bump rake to >= 12.3.3 for security reasons (see https://nvd.nist.gov/vuln/detail/CVE-2020-8130 for more information).

Fix build

Fix tests in the build.

v2.0.0

Changelog

Breaking Changes:

• Removes service_id and service_name attributes from invoice. #126

New Features:

• Adds support for new /v2 invoice endpoints #126

v1.15.0

New features:

• add support for /stats/usage_by_month #124

v1.5.0

• Adds support for the Edge Dictionary object and Dictionary Items.

v1.4.1

Includes User-Agent in the default headers