Fix language statistics on GitHub

[molnargab]

For some reason linguist considers the HTMLs in the build folder therefore making OpenCover 82% HTML.

82.21%  HTML
12.09%  C#
4.76%   C++
0.30%   XSLT
0.18%   Shell
0.15%   CSS
0.11%   C
0.09%   Visual Basic
0.07%   PowerShell
0.03%   Cucumber
0.00%   JavaScript

We could ignore this folder using .gitattributes.
Running linguist locally with .gitattributes shows somewhat the expected results:

69.10%  C#
27.10%  C++
1.70%   XSLT
1.02%   Shell
0.65%   C
0.27%   PowerShell
0.16%   Cucumber