We're now using GH actions for CI, travis be gone

.travis.yml

@@ -1,21 +0,0 @@
-language: rust
-rust:
-  - 1.39.0
-  - stable
-  - nightly
-cache: cargo
-
-env:
-  - COMMAND=""
-  - COMMAND=size_check
-
-matrix:
-  allow_failures:
-    - rust: nightly
-    - env: COMMAND=size_check
-  fast_finish: true
-
-install:
-  - rustup target add thumbv6m-none-eabi
-
-script: tools/check.py $COMMAND