Add python 3.7 build, improve CI, rework scoreboard

[pkulev]

Closes issue(s): [ #65 ]

Description of the changes:

• What's new:

  • python 3.7 build
  • updated container distro
  • travis uses commands for install and test in isolated environment (will be reworked later)
  • scoreboard now can be loaded from a corrupted CSV

• Refactoring:

  • scoreboard items is not generator now
  • remove scoreboard.ensure()
  • move IO related code to scoreboard._save() and scoreboard._load()

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@6b75bcb). Click here to learn what that means. The diff coverage is 100%.

@@           Coverage Diff            @@
##             master     #79   +/-   ##
========================================
  Coverage          ?   61.5%           
========================================
  Files             ?      42           
  Lines             ?    2390           
  Branches          ?       0           
========================================
  Hits              ?    1470           
  Misses            ?     920           
  Partials          ?       0

Impacted Files	Coverage Δ
xoinvader/scoreboard.py	100% <100%> (ø)
xoinvader/tests/test_scoreboard.py	100% <100%> (ø)

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