This version includes only internal changes. Here is a summary:
- No longer use Travis CI; instead we now use GitHub Actions.
- The website now uses the latest logo (#189).
- Polish documentation.
- Avoid error (https://bit.ly/avoid-cant-combine-spec-tbl-df):
Error: Can't combine `..1` <spec_tbl_df<>> and `..2` <tbl_df<>>.