Fix handling of installer version strings

[christophebedard]

The --all-versions option currently fails:

0.19.0

ERROR: Rosdep experienced an error: a bytes-like object is required, not 'str'
Please go to the rosdep page [1] and file a bug report with the stack trace below.
[1] : http://www.ros.org/wiki/rosdep

rosdep version: 0.19.0

Traceback (most recent call last):
  File "/home/chris/rosdep/src/rosdep2/main.py", line 144, in rosdep_main
    exit_code = _rosdep_main(args)
  File "/home/chris/rosdep/src/rosdep2/main.py", line 389, in _rosdep_main
    installer_version_strings = installer.get_version_strings()
  File "/home/chris/rosdep/src/rosdep2/platforms/debian.py", line 266, in get_version_strings
    version = output.splitlines()[0].split(' ')[1]
TypeError: a bytes-like object is required, not 'str'

I assume this (change to bytes) wasn't caught during the move to Python 3.

I also checked the other implementations of get_version_strings() and added calls to decode() where necessary. Otherwise it prints out b'1.2.3' instead of 1.2.3. I haven't tested nix or opkg, but it works fine for gem and apt-get.

[codecov-commenter]

Codecov Report

Merging #776 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #776      +/-   ##
==========================================
- Coverage   74.84%   74.81%   -0.04%     
==========================================
  Files          41       41              
  Lines        3220     3220              
==========================================
- Hits         2410     2409       -1     
- Misses        810      811       +1     

Impacted Files	Coverage Δ
src/rosdep2/platforms/debian.py	79.24% <0.00%> (ø)
src/rosdep2/platforms/gem.py	78.37% <0.00%> (ø)
src/rosdep2/platforms/nix.py	83.33% <0.00%> (ø)
src/rosdep2/platforms/openembedded.py	78.26% <0.00%> (ø)
src/rosdep2/shell_utils.py	97.72% <0.00%> (-2.28%)	⬇️

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[nuclearsandwich]

Thanks very much!