Support :same-site attribute

[jeroenvandijk]

Please find the following changes in this PR:

• [X] Update ring-core test dependency to make it aware of cookie attribute :same-site (link)
• [X] Add CI build script (as Travis.org seems to be EOL) #54
• [X] Add Failing test to showcase missing implementation #55
• [X] Add implementation for :same-site attribute
• [X] Fix CI (see builds of this fork)

I can split up these commits in seperate PR's if desired.

For full support we might also need to consider updating kerodon for when following links, this should have the same behaviour as redirects in peridot (see for more explaination this page under Differences Between the Strict and Lax SameSite Cookie Attributes

[danielcompton]

Sorry I missed this PR. Could we use GitHub Actions? Everything else looks ok.

[jeroenvandijk]

I removed the CI part. So this can be tackled separately.