updated travis badge

[AndrewSav]

Description

I'm using this PR to troubleshoot travis build issue. At the same time updating badge to the new travis address.

Fixes

Travis has moved to a different domain, the badge need to be updated to reflect that

Type of change

• [X] Build

Status

• [ ] Ready
• [X] In development
• [ ] Hold

Todo List

• [X ] Implementation

How has this been tested ?

There is no way to test it without creating PR, so the PR itself is going to be the test

[AndrewSav]

See also: https://github.com/mailserver2/mailserver/pull/37#issuecomment-901595790