Replace Travis with Cirrus

Travis has just stopped running... and wants me to select a plan to
re-enable it.

.cirrus.yml

@@ -0,0 +1,16 @@
+test_task:
+  container:
+    image: rust:latest
+  registry_cache:
+    folder: $CARGO_HOME/registry
+    fingerprint_script:
+      - rustc --version
+      - cat Cargo.toml
+  install_script:
+    - apt-get update && apt-get install -y --no-install-recommends binutils-dev cmake libcurl4-openssl-dev zlib1g-dev libdw-dev libiberty-dev python
+  test_script:
+    - cargo test
+    - cargo build --features=examples --examples
+  coverage_script:
+    - ci/upload-coverage
+  before_cache_script: rm -rf $CARGO_HOME/registry/index

.travis.yml

@@ -1,25 +0,0 @@
-language: rust
-cache: cargo
-script:
-  - |
-      cargo test &&
-      cargo build --features=examples --examples
-
-rust:
-  - stable
-  - beta
-matrix:
-  allow_failures:
-    - rust: beta
-
-addons:
-  apt:
-    packages:
-      - binutils-dev
-      - libcurl4-openssl-dev
-      - zlib1g-dev
-      - libdw-dev
-      - libiberty-dev
-
-after_success:
-  - ci/upload-coverage