Travis cleanup

This does 2 things

a) use `test-core.ini` file as is rather than hacking it

b) only test in python 2.6 and postgres 8.4 if on a release branch or commit ends with `++`

This should reduce the burden on travis and may speed up the time between pushes and the code being tested.  Hopefully it is also easier to read.

If this is accepted then we can look at cleaning up the 8.4 stuff as ckan should just work here without this magic tweaking.