revert taking the dependency on travis-cargo

It has issues with Coveralls:
huonw/travis-cargo#58

.travis.yml

@@ -1,38 +1,13 @@
-# Adapted from https://github.com/huonw/travis-cargo
-
-sudo: false
 language: rust
-# necessary for `travis-cargo coveralls --no-sudo`
-addons:
-  apt:
-    packages:
-      - libcurl4-openssl-dev
-      - libelf-dev
-      - libdw-dev
-      - binutils-dev # optional: only required for the --verify flag of coveralls
-
 rust:
   # Reenable stable after 1.12 is released.
   # - stable
   - beta
   - nightly
-
-# load travis-cargo
-before_script:
-  - |
-      pip install 'travis-cargo<0.2' --user &&
-      export PATH=$HOME/.local/bin:$PATH
-
-# the main build
-script: travis-cargo test
-after_success:
-  # measure code coverage and upload to coveralls.io (the verify
-  # argument mitigates kcov crashes due to malformed debuginfo, at the
-  # cost of some speed <https://github.com/huonw/travis-cargo/issues/12>)
-  - travis-cargo coveralls --no-sudo --verify
-
-# See https://github.com/huonw/travis-cargo/issues/23.
-env:
-  global:
-    # override the default `--features unstable` used for the nightly branch (optional)
-    - TRAVIS_CARGO_NIGHTLY_FEATURE=""
+# code coverage
+after_success: |
+  sudo apt-get install libcurl4-openssl-dev libelf-dev libdw-dev &&
+  wget https://github.com/SimonKagstrom/kcov/archive/master.tar.gz &&
+  tar xzf master.tar.gz && mkdir kcov-master/build && cd kcov-master/build &&
+  cmake .. && make && sudo make install && cd ../.. &&
+  kcov --coveralls-id=$TRAVIS_JOB_ID --exclude-pattern=/.cargo target/kcov target/debug/duct-*