Remove .travis.yml

Since we’re moving to GitLab CI, it should not be needed anymore.
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index 30f3f58..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,18 +0,0 @@
-sudo: false
-language: erlang
-otp_release:
-  - 20.1.6
-  - 20.0.4
-  - 19.0
-  - 18.3
-  - 17.3
-  - 17.1
-  - 17.0
-  - R16B03-1
-  - R15B03
-env: MAKE_TARGET=ci
-matrix:
-  include:
-    - otp_release: 20.1.6
-      env: MAKE_TARGET=ci-dialyze
-script: make $MAKE_TARGET