ci: remove Travis

Nixpkgs requires Nix 2.2 or greater to evaluate as of
96795314de,
and Travis does not appear to support this version yet, causing CI to
report failures on probably-good PRs. This can be reverted later on down
the line if we decide to have both GitHub Actions and Travis for
redundancy.

.travis.yml

@@ -1,11 +0,0 @@
-language: nix
-nix: 2.1
-sudo: true
-matrix:
-  include:
-    - name: cargo pedantry
-      script: nix-shell --run checkPhase -A mozilla-rust-overlay
-    - name: checkPhase
-      script: nix-shell --run checkPhase
-    - name: nix-build
-      script: nix-build ./default.nix -A ofborg.rs -A ofborg.php