Fix Package-Requires line

The existing line was not package.el-compliant, and rendered the
file unparseable by the `package-buffer-info` function. This
commit fixes the line, and additionally modifies the Carton file
to use the new "package-file" directive: using this directive,
Carton will read the package dependencies and version number from
the .el file just like package.el, meaning that changes need only
be made in one place.
Merge pull request #1 from purcell/fix-dependencies

Fix Package-Requires line, update Carton