Fix HTML report table not being scrollable (fix #186)

[devdanzin]

As shown in #186, the table containing metrics in HTML report doesn't scroll horizontally, so we can't see some metrics when displaying too many of them. This trivial PR changes the CSS so that the table has overflow set to scroll instead of hidden.

[codecov-commenter]

Codecov Report

Merging #208 (e40c7f4) into master (acf39d2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #208   +/-   ##
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  Coverage   95.46%   95.46%           
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  Files          26       26           
  Lines        1346     1346           
  Branches      287      287           
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  Hits         1285     1285           
  Misses         37       37           
  Partials       24       24           

[tonybaloney]

Thanks!