Fix coverage measurent

This PR fixes problems with the coverage measurements.
As a showcase of the fix, see PR https://github.com/OCA/department/pull/13, and the corresponding coverage report here: https://coveralls.io/builds/1078479

Two changes were made:
  * The server directory was moved from `$HOME/build/<owner>/<server-dir>` to `$HOME/<server-dir>`
  * The `.coveragerc` was improved.