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ARROW-2562: [CI] C++ and Rust code coverage using codecov.io

Collect C++ and Rust coverage info on Travis-CI build, and upload it to codecov.io.
The C++ coverage info is gathered from both the C++ and Python test suites.

Author: Antoine Pitrou <antoine@python.org>

Closes #2023 from pitrou/ARROW-2562-codecov-io and squashes the following commits:

8804ab5 <Antoine Pitrou> ARROW-2562:  C++ and Rust code coverage using codecov.io