readxl 1.5.0

• readxl should once again compile on Alpine Linux (#775).
• read_excel() and friends should no longer emit a spurious "Access is denied" warning when reading a file on a network drive and the user doesn't have access to a parent directory (#730).
• Minimum version of cpp11 has been bumped to v0.5.5, because cpp11 v0.5.4 (and only that exact version of cpp11) causes readxl to segfault when warning about coercion issues (#784).