Version increment to 0.4

Update version number of 0.4 for new release.
diff --git a/funcsigs/version.py b/funcsigs/version.py
index 6a35e85..896a370 100644
--- a/funcsigs/version.py
+++ b/funcsigs/version.py
@@ -1 +1 @@
-__version__ = "0.3"
+__version__ = "0.4"