[conda] Add pillow to dependencies

It was indirectly made available through beat/beat.core>
4 jobs for fix_dependencies in 8 minutes and 57 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #175963
docker-build
build_linux_36

00:05:30

54.0%
passed #175964
docker-build
build_linux_37

00:05:22

54.0%
passed #175961
macosx
build_macosx_36

00:08:57

54.0%
passed #175962
macosx
build_macosx_37

00:08:03

54.0%