Use high-quality icon for nuspecs

[ChrisMaddock]

See nunit/nunit3-vs-adapter#268

[CharliePoole]

@ChrisMaddock I merged this but the Travis build is failing. Two linux jobs fail. There seems to be an intermittent error in the messages we get for equal comparisons. It looks unrelated so I'm leaving this merged but you should take a look yourself to make sure it isn't due to any changes of yours.

[ChrisMaddock]

Did you re-run? They all look passed to me?

[ChrisMaddock]

Ah, I see - the master failed after merge. This is a semi-regular failure isn't it...I've always through it's an odd one to be sporadic! Only for the NET 3.5 framework too... I don't think changing the nuspec icon paths could have affected this - I'll rerun it now. 😄

1) Failed : NUnit.Framework.Constraints.EqualConstraintTests.FailsWithBadValues(double.PositiveInfinity,"Infinity")
  Expected string length 35 but was 62. Strings differ at index 13.
  Expected: "  Expected: 4\n  But was:  Infinity\n"
  But was:  "  Expected: 4 +/- 0.0050000000000000001d\n  But was:  Infinity\n"