[DRE-maint] Bug#1031518: gem2deb: prefer github.com tarballs in watch file when available

[Pirate Praveen]

Package: gem2deb
severity: wishlist

npm2deb does this already and we could do it too when gemspec mentions 
it the homepage as github. We might have to use the rubygems.org api to 
find homepage, like npm2deb does.

Also using api.github.com can help getting all tags instead of only the 
latest tags when there are too many frequent releases and we want an 
older version.

An example watch file is:

version=4
opts="searchmode=plain, \
  repacksuffix=+dfsg,repack,compression=xz,\
  dversionmangle=auto,\
  filenamemangle=s/.+\/v at ANY_VERSION@/@PACKAGE at -$1\.tar\.gz/" \
https://api.github.com/repos/chartjs/Chart.js/releases \
https://api.github.com/repos/chartjs/Chart.js/tarball/v@ANY_VERSION@ 
group