Remove bin/erl before replacing it

In some erlang distributions (e.g. brew) the ERTS files do not have 
write permissions set.  When this is the case the file:write_file will
fail with error 'eacces'.

This commit removes 'bin/erl' before attempting to write the new file.
Merge pull request #53 from zerotao/master

Remove bin/erl from the local ERTS copy before writing a the new version.