Support dplyr 1.0.0

[chezou]

RTD has an issue with dplyr 1.0.0 which is coming major version soon. After releasing the 1.0.0, we should ensure tibble data frames as a table and database list.

== CHECK RESULTS ========================================

*   checking tests ...
    ```
     ERROR
    Running the tests in ‘tests/testthat.R’ failed.
    Last 13 lines of output:
      >
      > test_check("RTD")
      ── 1. Error: list_tables works with mock (@test-table.R#18)
───────────────────
      `new` must be a tibble
      Backtrace:
       1. testthat::expect_equal(dplyr::count(tables)$n, 2)
       4. dplyr::count(tables)
       5. dplyr::dplyr_reconstruct(out, x)

      ══ testthat results
═══════════════════════════════════════════════════════════
      [ OK: 26 | SKIPPED: 0 | WARNINGS: 0 | FAILED: 1 ]
      1. Error: list_tables works with mock (@test-table.R#18)

      Error: testthat unit tests failed
      Execution halted
    ```

[chezou]

dplyr 1.0.0 has been released. Now is the time to apply this patch.
https://github.com/tidyverse/dplyr/releases/tag/v1.0.0

[chezou]

Not sure why, but Travis CI badge hasn't been updated, but CI passed. Will merge.