ci: remove Travis CI (#421)

Removing "dead code"

.travis.yml

@@ -1,68 +0,0 @@
-dist: xenial
-sudo: required
-cache: cargo
-language: rust
-addons:
-  apt:
-    packages:
-      - kcov
-      - libcurl4-openssl-dev
-      - libelf-dev
-      - libdw-dev
-      - binutils-dev
-      - cmake
-    sources:
-      - kalakris-cmake
-      # The version of kcov shipped with Xenial (v25) doesn't support the
-      # --verify option that `cargo coveralls` passes. This PPA has a more
-      # up-to-date version. It can be removed if Ubuntu ever ships a newer
-      # version, or replaced with another PPA if this one falls out of date.
-      - sourceline: ppa:sivakov512/kcov
-
-rust:
-  - stable
-
-before_script:
-  # Travis installs rust with a non-default "minimal" profile, and advises us
-  # to add clippy manually:
-  - rustup component add clippy
-  # Unfortunately, this method often breaks on the nightly channel, where the
-  # most recent build might not have all the optional components.
-  # We explicitly specify `--profile default` to obtain the most recent nightly
-  # that has rustfmt (we don't care if it's a week old, as we need it only for
-  # an experimental flag):
-  - rustup toolchain install nightly --profile default
-
-  - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
-  - export PATH=$HOME/.cargo/bin:$PATH
-  # `cargo install` fails if the specified binary is already installed, and
-  # doesn't yet support a `--if-not-installed` option [0], so for now assume
-  # failures mean the package is already installed. If installation truly
-  # failed, the build will fail later anyway, when we try to use the installed
-  # binary. Note that `cargo install --force` is not a solution, as it always
-  # rebuilds from scratch, ignoring the cache entirely.
-  #
-  # [0]: https://github.com/rust-lang/cargo/issues/2082
-  - cargo install cargo-update || echo "cargo-update already installed"  # for `cargo install-update`
-  - cargo install cargo-travis || echo "cargo-travis already installed"  # for `cargo coveralls`
-  - cargo install-update -a  # updates cargo-travis, if the cached version is outdated
-
-script:
-  # Clippy must be run first, as its lints are only triggered during
-  # compilation. Put another way: after a successful `cargo build`, `cargo
-  # clippy` is guaranteed to produce no results. This bug is known upstream:
-  # https://github.com/rust-lang/rust-clippy/issues/2604.
-  - travis-cargo clippy -- --all-targets --all-features -- -D warnings
-  - travis-cargo build
-  - travis-cargo test
-  - travis-cargo test -- all-features
-  # The license_template_path setting we use to verify copyright headers is
-  # only available on the nightly rustfmt.
-  - cargo +nightly fmt -- --check --config-path <(echo 'license_template_path = "HEADER"')
-
-after_success:
-  - cargo coveralls --verbose
-
-env:
-  global:
-    - TRAVIS_CARGO_NIGHTLY_FEATURE=""