all: remove travisci

[leighmcculloch]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

Summary

Remove .travis.yml and support for testing this repository on Travis CI.

Goal and scope

Simplify the CI files we need to manage and maintain. @bartekn pointed out in #1542 (comment) that we don't use TravisCI anymore and we shouldn't keep it around and expect it to be kept up to date. For contributors who fork the repository and have TravisCI installed as an application in their GitHub account TravisCI also runs in addition to CircleCI because this file is present, which is distracting given it's not the build system we're relying on.

Summary of changes

• Remove .travis.yml.

Known limitations & issues

N/A

What shouldn't be reviewed

N/A