[melodic] Fix travis CI

[seanyen]

Fix travis CI build for melodic. The change is adapted from tork-a/visualization_rwt@4a93906.

One standing error is test_encode_decode_cv2_compressed failure, which is also reproducible on Mpr__vision_opencv__ubuntu_bionic_amd64 build.

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ERROR: test_encode_decode_cv2_compressed (conversions.TestConversions)
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Traceback (most recent call last):
  File "/home/travis/catkin_ws/src/vision_opencv/cv_bridge/test/conversions.py", line 64, in test_encode_decode_cv2_compressed
    compress_rosmsg = cvb_en.cv2_to_compressed_imgmsg(original, f)
  File "/home/travis/catkin_ws/src/vision_opencv/cv_bridge/python/cv_bridge/core.py", line 219, in cv2_to_compressed_imgmsg
    cmprs_img_msg.data = np.array(cv2.imencode(ext_format, cvim)[1]).tostring()
error: /build/opencv-L2vuMj/opencv-3.2.0+dfsg/modules/imgcodecs/src/loadsave.cpp:783: error: (-2) could not find encoder for the specified extension in function imencode

[seanyen]

After checking the failing format, it is jp2 in test_encode_decode_cv2_compressed. Perhaps we should consider cherry-pick #242 too?

[seanyen]

@mjcarroll Hope I am pinging the correct person. There are some PRs created recently for bringing up vision_opencv for Windows build, which is verified on https://aka.ms/ros project. Hope I can get some feedbacks and see how to make those changes merged back and available for Windows users. Thanks!

Here is a list for all related PRs for quick reference:
#269 (This one is nice to have, so we know no test regression seen from the Windows changes.)
#267
#266
#265

[seanyen]

@mjcarroll Sorry to ping you again. Any chance to take a look and see how I can move the Windows related PR forward?