Require B::Size2 >= 2.07

Readonly our @FOO => qw(foo bar) causes segmentaion fault
fixed in 2.06
https://github.com/gfx/p5-B-Size2/issues/2

B::Size2::Terse::package_size die when checking Math::BigFloat and Math::BigInt
fixed in 2.07
https://github.com/gfx/p5-B-Size2/issues/3