build: remove travis config

as I did not find it got referenced in the PR builds

.travis.yml

@@ -1,70 +0,0 @@
-dist: xenial
-services:
-- docker
-
-env: # for allow_failures: https://docs.travis-ci.com/user/customizing-the-build/
-matrix:
-  include:
-
-  - language: go
-    name: "Build in Docker (docs/installation.rst)"
-    script:
-    - sudo docker build -t zrepl_build -f build.Dockerfile .
-    - |
-      sudo docker run -it --rm \
-        -v "${PWD}:/go/src/github.com/zrepl/zrepl" \
-        --user "$(id -u):$(id -g)" \
-        zrepl_build make vendordeps release
-
-  - &zrepl_build_template
-    language: go
-    go_import_path: github.com/zrepl/zrepl
-    before_install:
-        - wget https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protoc-3.6.1-linux-x86_64.zip
-        - echo "6003de742ea3fcf703cfec1cd4a3380fd143081a2eb0e559065563496af27807  protoc-3.6.1-linux-x86_64.zip" | sha256sum -c
-        - sudo unzip -d /usr protoc-3.6.1-linux-x86_64.zip
-        - ./lazy.sh godep
-        - make vendordeps
-    script:
-      - make
-      - make vet
-      - make test
-      - make lint
-      - make artifacts/zrepl-freebsd-amd64
-      - make artifacts/zrepl-linux-amd64
-      - make artifacts/zrepl-darwin-amd64
-    go:
-    - "1.11"
-
-  - <<: *zrepl_build_template
-    go:
-    - "1.12"
-
-  - <<: *zrepl_build_template
-    go:
-    - "master"
-
-  - &zrepl_docs_template 
-    language: python
-    python:
-    - "3.4"
-    install:
-    - sudo apt-get install libgirepository1.0-dev
-    - pip install -r docs/requirements.txt
-    script:
-    - make docs
-  - <<: *zrepl_docs_template
-    python:
-    - "3.5"
-  - <<: *zrepl_docs_template
-    python:
-    - "3.6"
-  - <<: *zrepl_docs_template
-    python:
-    - "3.7"
-  
-
-  allow_failures:
-  - <<: *zrepl_build_template
-    go:
-    - "master"