make das_create_json_maps less environment specific

* make the bin/das_create_json_maps to correctly identify dasroot
  - try importing DAS to get dasroot, if unsuccessful use $PWD
  - now it will work even when PYTHONPATH is unavailable, as pip or system-wide install
* update travis config accordingly

comment: one may introduce a separate (optional) argument for dasroot, but I see no such need for now
Merge pull request #4137 from dmwm/dasroot_fix

make das_create_json_maps less environment specific