Fix emscripten-version parsing (#902)

See https://github.com/emscripten-core/emscripten/pull/15144
diff --git a/emsdk.py b/emsdk.py
index 8d859eb..ea516b5 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -2204,7 +2204,7 @@
   version_file = os.path.join(emscripten_root, 'emscripten-version.txt')
   with open(version_file) as f:
     version = f.read().strip()
-    version = version.strip('"').split('.')
+    version = version.strip('"').split('-')[0].split('.')
     return [int(v) for v in version]