Changed pypy_architecture reference to pyston alias

plugins/python-build/share/python-build/pyston-0.5.1

@@ -1,4 +1,4 @@
-case "$(pypy_architecture 2>/dev/null || true)" in
+case "$(pyston_architecture 2>/dev/null || true)" in
 "linux64" )
   install_package "pyston-0.5.1-linux64" "https://github.com/dropbox/pyston/releases/download/v0.5.1/pyston-0.5.1-linux64.tar.gz#4b0d2ad2c19f6393b79adbb9312649d38cb8cb3daf0c8fdf8631465c7761bb79" "pyston" verify_py27
   # disabling ensurepip avoids having installation aborted because of a coredump