fixed double /var/tmp/ download prefix (bnc#847794)

- zypp::filesystem::TmpDir::defaultLocation() already contains
  /var/tmp/ prefix, do not use it twice (bnc#847794)
- 3.1.3
Merge pull request #14 from yast/var_tmp_fix

Fixed double /var/tmp/ download prefix (bnc#847794)