Enable export substitution for _version.py

I'm not sure why this was missed, but when you install versioneer, it
adds these attributes so that things like version/commit hash/etc. are
correct when you make a git archive. Without this substitution,
`test_git_revision` fails because there is no revision info in the
archive tarball.

.gitattributes

@@ -0,0 +1 @@
+distributed/_version.py export-subst