Fpm: loosen json dependency to accept > 1.7.7 up to < 3.0

[jsvd]

Looking at the breaking changes in json 2 it seems that we can allow fpm to depend on json ["> 1.7", "< 3.0"] which would make fpm work on rubies from 1.8 to 2.6.

[jordansissel]

1600 merged. I'll work on a new release sometime today.

[xli1996]

@jordansissel Hi, I wonder if it possible to release a new version since JSON has a cve(https://nvd.nist.gov/vuln/detail/CVE-2020-10663). It would be good to release a new version with json dependency up to 3.0. Many thanks.