Do not replace '-' with '.' in the version string

This prevents version strings like 0.15-rc1.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>

stgit/version.py

diff --git a/stgit/version.py b/stgit/version.py
index d57053d..948a70b 100644
--- a/stgit/version.py
+++ b/stgit/version.py
@@ -21,7 +21,7 @@
         raise VersionUnavailable(str(e))
     if dirty:
         v += '-dirty'
-    return re.sub('-', '.', utils.strip_prefix('v', v))
+    return utils.strip_prefix('v', v)
 
 def builtin_version():
     try: