Exclude _version.py from coverage report (#988)

We vendor the code in _version.py from scipy. It is thus not necessary
for us to have tests for it. Still, it was included in the coverage
report, resulting in artificially lowered test coverage. This fix
removes the module from being counted for coverage.

.coveragerc

@@ -1,4 +1,4 @@
 [run]
 omit = 
        skorch/tests/*
-
+       skorch/_version.py