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ARROW-2477: [Rust] Set up code coverage in CI

It may require some setup on the coveralls.io. I'm just giving it a blind try right now.

Author: Chao Sun <sunchao@apache.org>

Closes #1942 from sunchao/rust-code-coverage and squashes the following commits:

3c351ca8 <Chao Sun> fix
061b66f2 <Chao Sun> try again
0142a71a <Chao Sun> try again
bd53614f <Chao Sun> try again
697adc7b <Chao Sun> more fix
81996e83 <Chao Sun> more fix
14e7a590 <Chao Sun> use travis-cargo
5af1bd07 <Chao Sun> ARROW-2477:  Set up code coverage in CI