Use an unreleased cargo coveralls, which works with the modern Carg…

…o.lock format

See roblabla/cargo-travis#66

.travis.yml

@@ -44,7 +44,7 @@ before_script:
   #
   # [0]: https://github.com/rust-lang/cargo/issues/2082
   - cargo install cargo-update || echo "cargo-update already installed"  # for `cargo install-update`
-  - cargo install cargo-travis || echo "cargo-travis already installed"  # for `cargo coveralls`
+  - cargo install --git https://github.com/roblabla/cargo-travis --branch cargo-metadata || echo "cargo-travis already installed"  # for `cargo coveralls`
   - cargo install-update -a  # updates cargo-travis, if the cached version is outdated
 
 script: