Fix travis code coverage

The installation steps for cargo-travis was deleted by mistake.
The CI jobs were succeeding because it was using cache, but recently
started to fail because the cache is gone. This fixes it.

.travis.yml

@@ -16,6 +16,11 @@ addons:
 rust:
   - nightly
 
+before_script:
+  - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
+  - cargo install cargo-travis || echo "Skipping cargo-travis installation as it is already cached"
+  - export PATH=$HOME/.cargo/bin:$PATH
+
 script:
   - cargo build
   - cargo test