changes Group move ctor to work with gcc-4.8
changes Group move ctor to work with gcc-4.8

If this were a big change I wouldn't do it, but all it needs is changing
a{b} to a(b) in one spot, so why not.

jkhoogland/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools