.travis.yml: Simplify the PostGIS database creation
.travis.yml: Simplify the PostGIS database creation

This switches to using the method suggested here:

  http://docs.travis-ci.com/user/using-postgresql/#Using-PostGIS

hzj123/56th,geoffkilpin/pombola,patricmutwiri/pombola,ken-muturi/pombola,patricmutwiri/pombola,hzj123/56th,mysociety/pombola,hzj123/56th,ken-muturi/pombola,mysociety/pombola,mysociety/pombola,geoffkilpin/pombola,ken-muturi/pombola,hzj123/56th,mysociety/pombola,ken-muturi/pombola,patricmutwiri/pombola,geoffkilpin/pombola,hzj123/56th,geoffkilpin/pombola,ken-muturi/pombola,geoffkilpin/pombola,mysociety/pombola,hzj123/56th,mysociety/pombola,patricmutwiri/pombola,geoffkilpin/pombola,patricmutwiri/pombola,ken-muturi/pombola,patricmutwiri/pombola