pyenv-version-file-write: do not use rm, but clobber the file

With e.g. /usr/local/bin/.python-version owned by some user, `pyenv
local foo` would fail, if the user has no permissions for
`/usr/local/bin`, but only the `.python-version` file.
Merge pull request #230 from blueyed/version-file-write-no-rm

pyenv-version-file-write: do not use rm, but clobber the file