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ARROW-4537: [CI] Suppress shell warning on travis-ci

Enclose optional shell variable names by double quotations to suppress shell warnings that is like below:

```
+'[' == 1 ']'
/home/travis/build/apache/arrow/ci/travis_before_script_cpp.sh: line 81: [: ==: unary operator expected
```

Author: Kenta Murata <mrkn@mrkn.jp>

Closes #3623 from mrkn/suppress_shell_warning_on_ci and squashes the following commits:

4db2e53d <Kenta Murata> Suppress shell warning on travis-ci