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, e.g. in pip or system-wide installs
* update travis config accordingly

comment: one may introduce a separate (optional) argument for dasroot but this would require changing existing scripts (including das.spec), so I do see no such need for now