bchunk: fix recent CVEs, simplify installPhase
For details, see:
https://bugs.gentoo.org/635898
https://nvd.nist.gov/vuln/detail/CVE-2017-15953
https://nvd.nist.gov/vuln/detail/CVE-2017-15954
https://nvd.nist.gov/vuln/detail/CVE-2017-15955
https://github.com/extramaster/bchunk/issues/2
https://github.com/extramaster/bchunk/issues/3
https://github.com/extramaster/bchunk/issues/4