chore: Remove Travis CI config

Travis is now a paying service, so there’s no way this will ever work again.

.travis.yml

@@ -1,39 +0,0 @@
-language: python
-matrix:
-    include:
-        - os: linux
-          python: 3.5
-          dist: xenial
-        - os: linux
-          python: 3.6
-          dist: xenial
-        - os: linux
-          python: 3.7
-          dist: xenial
-        - os: linux
-          python: 3.8-dev
-          dist: xenial
-        # shamelessly stolen from https://github.com/pyload/pyload
-        - os: osx
-          language: sh
-          env:
-            - HOMEBREW_NO_INSTALL_CLEANUP=1
-            - HOMEBREW_NO_ANALYTICS=1
-          before_cache:
-            - rm -f "$HOME/Library/Caches/pip/log/debug.log"
-          cache:
-            directories:
-              - "$HOME/Library/Caches/pip"
-          addons:
-            homebrew:
-              packages: python3
-          before_install:
-            - python3 -m pip install --upgrade virtualenv
-            - virtualenv -p python3 --system-site-packages "$HOME/venv"
-            - source "$HOME/venv/bin/activate"
-install:
-    - pip install .[tests]
-    - pip install coveralls
-script: pytest
-after_success:
-  - coveralls