Added netcore2.2 build

[fontmaniac]

• Added netcore2.2 build in a manner similar to netcore2.0
• Fixed net35 build along with HelloTriangle.WPF sample in VS2017 on WIndows7
• Locked Travis.CI to use Trusty, as it was trying (and failing) on Xenial.

[codecov[bot]]

Codecov Report

Merging #135 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #135   +/-   ##
=======================================
  Coverage   92.14%   92.14%           
=======================================
  Files          91       91           
  Lines       27663    27663           
  Branches     1938     1938           
=======================================
  Hits        25490    25490           
  Misses       1926     1926           
  Partials      247      247

Flag	Coverage Δ
#WGL_net35_x64_Release	92.22% <ø> (ø)	:arrow_up:
#WGL_net35_x86_Release	90.97% <ø> (ø)	:arrow_up:
#WGL_net461_x64_Release	92.22% <ø> (+0.63%)	:arrow_up:
#WGL_net461_x86_Release	90.71% <ø> (ø)	:arrow_up:

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