Travis job optimisations

[NejcZdovc]

What was done:

• optimization of travis file
• added allowed failures (webdriver tests). This means that PR will now actually get green thick if required jobs are passing.
• added fast check. This means that travis will only wait for required jobs (unit test, linter and codecov) and as soon as this are done will return response to the PR
• fixed codecov test

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header