Cannot import matplotlib.pyplot on kinetic and melodic travis tests about jsk_apc

[pazeshun]

As the error message says, python-tk is not installed on jskrobotics/jsk_apc:<kinetic/melodic>-latest-testing docker image:

$ dpkg -l python-tk
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                        Version            Architecture       Description
+++-===========================-==================-==================-===========================================================
un  python-tk                   <none>             <none>             (no description available)

By contrast, python-tk is installed on jskrobotics/jsk_apc:indigo-latest-testing image:

# dpkg -l python-tk
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                        Version            Architecture       Description
+++-===========================-==================-==================-===========================================================
ii  python-tk                   2.7.5-1ubuntu1     amd64              Tkinter - Writing Tk applications with Python

[pazeshun]

python-tk is dropped from osrf/ros:<kinetic/melodic>-desktop-<xenial/bionic>, which is original image of jskrobotics/jsk_apc:<kinetic/melodic>-latest-testing:
osrf/docker_images#423

[pazeshun]

Dropping python-tk (and other "Recommends" dependencies) was what the maintainer expected: osrf/docker_images#423 (comment)
We have to install lacking dependencies explicitly -> #2719