(textproc/R-readxl) Updated 1.3.1 to 1.4 3

# readxl 1.4.3

This release contains no user-facing changes.

# readxl 1.4.2

This release contains no user-facing changes.

* We embed a development version of libxls (<https://github.com/libxls/libxls>), which is based on the most recent released version, v1.6.2.
The reason for embedding a development version is to ship a version of libxls that incorporates the fix for this CVE (#679):

  - [CVE-2021-27836](https://nvd.nist.gov/vuln/detail/CVE-2021-27836)
  
* readxl no longer declares the use of C++11.

* readxl should once again compile on Alpine Linux.

* Other small readxl-specific patches have been made to the embedded libxls code to comply with CRAN requests, such as avoiding the use of `sprintf()`.

# readxl 1.4.1

Help files below `man/` have been re-generated, so that they give rise to valid HTML5. (This is the impetus for this release, to keep the package safely on CRAN.)

# readxl 1.4.0

This release is mostly about substantial internal changes that should not be
noticeable to most users (but that set the stage for future work):

* Updating the embedded version of libxls (more below)
* Switching from Rcpp to cpp11 (more below)
* Refactoring to reduce duplication between the `.xls` and `.xlsx` branches

However, there are a few small features / bug fixes:

* "Date or Not Date": The classification of number formats as being datetime-ish
  is more sophisticated and should no longer be so easily fooled by, e.g.,
  colours or currencies. This affects cell and column type guessing, hopefully
  for the better (#388, #559, @nacnudus, @reviewher).

* Cell location is determined more robustly in `.xlsx` files, guarding against
  the idiosyncratic way in which certain 3rd party tools include (or, rather,
  do not include) cell location in individual cell nodes (#648, #671).

* Warning messages for impossible dates are more specific.
  Unsupported dates prior to 1900 have their own message now, instead of being
  lumped in with dates on the non-existent day of February 29, 1900
  (#551, #554, @cderv).

## Dependency and licensing changes

* readxl is now licensed as MIT (#632).

* readxl now states its support for R >= 3.4 explicitly. 
  Why 3.4? Because the [tidyverse policy](https://www.tidyverse.org/blog/2019/04/r-version-support/)
  is to support the current version, the devel version, and four previous
  versions of R.
  It was necessary to introduce a minimum R version, in order to state a minimum
  version for a package listed in `LinkingTo`.

* readxl embeds libxls v1.6.2 (the previous release embedded v1.5.0).
  The libxls project is hosted at <https://github.com/libxls/libxls> and you can
  learn more about the cumulative changes in its release notes:

  - [v1.6.2](https://github.com/libxls/libxls/releases/tag/v1.6.2)
  - [v1.6.1](https://github.com/libxls/libxls/releases/tag/v1.6.1)
  - [v1.6.0](https://github.com/libxls/libxls/releases/tag/v1.6.0)
  - [v1.5.3](https://github.com/libxls/libxls/releases/tag/v1.5.3)
  - [v1.5.2](https://github.com/libxls/libxls/releases/tag/v1.5.2)
  - [v1.5.1](https://github.com/libxls/libxls/releases/tag/v1.5.1)
  
* readxl has switched from Rcpp to cpp11 and now requires C++11 (#659,
  @sbearrows).

* The minimum version of tibble has been bumped to 2.0.1 (released 2019-01-12),
  completing the transition to an approach to column name repair used across the
  tidyverse.
