Update .travis.yml to match latest tarpaulin snippet

.travis.yml

@@ -7,16 +7,24 @@ addons:
             - libssl-dev
 cache: cargo
 rust:
-  - nightly
-  - beta
   - stable
+  - beta
+  - nightly
+matrix:
+  allow_failures:
+    - rust: nightly
+
+before_cache: |
+  if [[ "$TRAVIS_RUST_VERSION" == nightly ]]; then
+    RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin -f
+  fi
+
 script:
 - cargo clean
 - cargo build
 - cargo test
 
 after_success: |
-  if [[ "$TRAVIS_RUST_VERSION" == stable ]]; then
-    RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin
+  if [[ "$TRAVIS_RUST_VERSION" == nightly ]]; then
     cargo tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID
   fi