Fix crash tensor.roll(Var, iscalar) reported by Jeremiah Lowin.
Added link to Nicolas Pinto ebuild on gentoo.
Fix memory leak in HostFromGpu when allow_gc=False!

fix gh-1154
Merge pull request #1173 from nouiz/fix_crash_roll

Fix crash tensor.roll(Var, iscalar) reported by Jeremiah Lowin.