Test beta and nightly on CI

.travis.yml

@@ -1,16 +1,31 @@
 language: "rust"
 cache: "cargo"
+rust:
+  - stable
+  - beta
+  - nightly
+jobs:
+  allow_failures:
+    - rust: nightly
+  fast_finish: true
+
 before_script:
-  - "rustup component add rustfmt"
-  - "rustup component add clippy"
-  - "cargo install --force cargo-audit"
-  - "cargo generate-lockfile"
+  - rustup component add rustfmt
+  - rustup component add clippy
+  - |
+      if [ "$TRAVIS_RUST_VERSION" == "stable" ]; then
+        cargo install --force cargo-audit
+      fi
+  - cargo generate-lockfile
 script:
-  - "cargo audit"
-  - "cargo fmt --all -- --check"
-  - "cargo clippy --all-targets --all-features -- -D warnings"
-  - "cargo build --verbose"
-  - "cargo test --verbose"
+  - |
+      if [ "$TRAVIS_RUST_VERSION" == "stable" ]; then
+        cargo audit
+      fi
+  - cargo fmt -- --check
+  - cargo clippy --all-targets --all-features -- -D warnings
+  - cargo build --verbose
+  - cargo test --verbose
 if: "type != push OR (tag IS blank AND branch = master)"
 notifications:
   email: false