test i128 only on newer rustc

.travis.yml

@@ -1,6 +1,8 @@
 language: rust
 rust:
   - 1.15.0
+  - 1.20.0
+  - 1.25.0
   - stable
   - beta
   - nightly

ci/rustup.sh

@@ -5,7 +5,7 @@
 set -ex
 
 export TRAVIS_RUST_VERSION
-for TRAVIS_RUST_VERSION in 1.15.0 stable beta nightly; do
+for TRAVIS_RUST_VERSION in 1.15.0 1.20.0 1.25.0 stable beta nightly; do
     run="rustup run $TRAVIS_RUST_VERSION"
     $run cargo build --verbose
     $run $PWD/ci/test_full.sh

ci/test_full.sh

@@ -18,5 +18,15 @@ for feature in rand serde; do
   cargo test --verbose --no-default-features --features="$feature"
 done
 
-cargo build --all-features
-cargo test --all-features
+# test `i128` and all features together
+if [[ "$TRAVIS_RUST_VERSION" =~ ^(nightly|beta|stable)$ ]]; then
+  cargo build --verbose --features=i128
+  cargo test --verbose --features=i128
+
+  cargo build --all-features
+  cargo test --all-features
+else
+  # all except `i128`
+  cargo build --features="rand serde"
+  cargo test --features="rand serde"
+fi