readxl 1.4.4

This release contains no user-facing changes.

• readxl embeds libxls v1.6.3, with release notes at
  https://github.com/libxls/libxls/releases/tag/v1.6.3.
  This version of libxls fixes several vulnerabilities.
• Other small internal changes have been made to comply with CRAN requests,
  such as avoiding the use of Rf_StringTrue().