upgrade apache POI 3.14 -> 3.15
upgrade apache POI 3.14 -> 3.15

This provides bugfixes and a security update

see:
  - https://poi.apache.org/changes.html

mvdstruijk/flamingo,flamingo-geocms/flamingo,B3Partners/flamingo,B3Partners/flamingo,mvdstruijk/flamingo,flamingo-geocms/flamingo,mvdstruijk/flamingo,flamingo-geocms/flamingo,B3Partners/flamingo,B3Partners/flamingo,flamingo-geocms/flamingo,mvdstruijk/flamingo