trying out some improvements to hopefully speed up the travis build

.travis.yml

@@ -6,6 +6,10 @@ addons:
     packages:
       - libssl-dev
 cache: cargo
+env:
+  - ELEFREN_FEATURES="--features toml"
+  - ELEFREN_FEATURES="--features all"
+  - ELEFREN_FEATURES=""
 rust:
   - stable
   - beta
@@ -16,9 +20,7 @@ before_script:
 script:
   - |
     if [[ "$TRAVIS_RUST_VERSION" == "nightly"* ]]; then cargo fmt --all -- --check; fi
-  - cargo clean && cargo build
-  - cargo clean && cargo build --features toml
-  - cargo clean && cargo build --features all
+  - cargo clean && cargo build $ELEFREN_FEATURES
   - cargo clean && cargo test --features all
 after_success: |
   if [[ "$TRAVIS_RUST_VERSION" == stable ]]; then