fixed failing test Config->testVarReplacement() on windows, referencing ...

...https://github.com/composer/composer/commit/449f8165ef64829dc34be836235ceb74d9227dd7 and issue #3060

1) Composer\Test\ConfigTest::testVarReplacement
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/'
+'C:\Users\koch/'