Properly test code on oldest supported rust version

This PR fixes #28. We don't currently verify `human-panic`
properly works anymore. So this adds running tests on the
oldest version we support (which is 1.25.*)

.travis.yml

@@ -8,8 +8,7 @@ script: |
 matrix: # additional tests
   include:
   - rust: nightly
-    script: |
-       cargo test --all --features nightly
+    script: cargo test --all --features nightly
   - rust: 1.29.1
     env: CLIPPY=YESPLEASE
     before_script: rustup component add clippy-preview
@@ -18,4 +17,6 @@ matrix: # additional tests
     env: RUSTFMT=YESPLEASE
     before_script: rustup component add rustfmt-preview
     script: cargo fmt --all -- --check
+  - rust: 1.25
+    script: cargo test --all --verbose
 cache: cargo