Reset build directory between tests

This fixes:

    error[E0464]: multiple matching crates for `indoc`
      --> tests/ui-stable/no-arguments.rs:10:1
       |
    10 | extern crate indoc;
       | ^^^^^^^^^^^^^^^^^^^
       |
       = note: candidates:
               crate `indoc`: target/debug/deps/libindoc-71f336784875944c.rlib
               crate `indoc`: target/debug/deps/libindoc-865d3d42afd5a45d.rlib

    error[E0463]: can't find crate for `indoc`

Author: dtolnay

.travis.yml

@@ -13,5 +13,7 @@ matrix:
 
 script:
   - cargo build --features "${FEATURES:-}" --verbose
+  - cargo clean
   - cargo test --features "${FEATURES:-}" --verbose
+  - cargo clean
   - cargo test --features "${TEST_FEATURES:-}" --manifest-path tests/Cargo.toml --verbose