Include additional checks in CI

Update .travis.yml to include tests if the crate builds with the `alloc` feature enabled.
These scripts are only executed if the rust version is != 1.31.0.
These changes were originally included in pyfisch#147 but with incorrect YAML syntax.

.travis.yml

@@ -13,10 +13,16 @@ before_script:
   - rustup target add thumbv7em-none-eabihf     # Any target that does not have a standard library will do
 script:
   - cargo fmt --all -- --check
-  - (rustup component add clippy && cargo clippy --all --all-features -- -D clippy::all) || exit 0
+  - (rustup component add clippy && cargo clippy --all -- -D clippy::all) || true
   - cargo build
   - cargo test
-  - cargo build --no-default-features --target thumbv7em-none-eabihf # Test we can build a platform that does not have std.
+  - >
+    [[ $TRAVIS_RUST_VERSION == "1.31.0" ]]
+    || cargo build --no-default-features --features alloc --target thumbv7em-none-eabihf
+    # Test we can build a platform that does not have std.
   - cargo test --no-default-features --lib --tests # Run no_std tests
+  - >
+    [[ $TRAVIS_RUST_VERSION == "1.31.0" ]]
+    || cargo build --no-default-features --features alloc
   - cargo build --features unsealed_read_write # The crate should still build when the unsealed_read_write feature is enabled.
   - cargo build --no-default-features --features unsealed_read_write # The crate should still build when the unsealed_read_write feature is enabled and std disabled.