build: update rpmbuild .spec file

Rename the package to iojs.  No Conflicts: header is necessary because
the package was already marked as conflicting with the Fedora nodejs
package.

R=@rvagg, possibly useful for iojs/build#18.
In this case we should note it somewhere like npm guys [did](https://github.com/npm/npm/blob/e24926268b2d2220910bc81cce6d3b2e08d94eb1/doc/misc/npm-faq.md#if-npm-is-an-acronym-why-is-it-never-capitalized). Maybe in readme.