Make DFS tests pass on 32-bit machines.

simple_inclist uses Ints for vertices; on 32-bit machines, Int == Int32.
But test/dfs.jl hard-coded the use of Int64s for test data, which caused
type errors.
Merge pull request #66 from pozorvlak/32bit_dfs_test

Make DFS tests pass on 32-bit machines.