Fix issue where non-string '$ref' properties were assumed to be references

Prior to this patch, z-schema would assume any '$ref' property on an object was
a reference.  When this happened, numerous locations would fail due to this
assumption.  I updated the locations I could replicate.
move self variable initialization to top of function
Merge pull request #46 from fsbdev/patch-1

move self variable initialization to top of function
Merge pull request #42 from whitlockjc/master

Fix issue where non-string '$ref' properties were assumed to be referenc...
new submodule version
mixed failing ipv4 test
issue #37
issue #40
issue #43
issue #44
issue #45
Version 2.4.8