Support more Gentoo package managers

* Portage
* Paludis
* PkgCore
Merge pull request #1801 from cheba/gentoo-pakcage-managers

Support more Gentoo package managers
remove git requirement from default libs set for ruby on gentoo, update #1801
add a warning for to short timeout in checking for remote files, fix #1814
add --2.0 flag support, closes #1826
fallback to default ruby if selected one got upgraded, closes #1528
add rvm fix-permissions, closes #1784