[cherry-pick #84] Upgrade NUnit from v2.6.2 to v2.6.3 to align with Dynamo Core

[alfarok]

Purpose

Related to: Original RTF PR Dynamo-8914 D4R-2068 QNTM-4186

This PR upgrades NUnit from v2.6.2 to v2.6.3 to align with Dynamo Core. Once this PR is reviewed and if deemed appropriate to merge I will cherry-pick to all required branches.

Declarations

Check these if you believe they are true

• [x] The code base is in a better state after this PR
• [x] Is documented according to the standards
• [x] The level of testing this PR includes is appropriate
• [x] User facing strings, if any, are extracted into *.resx files
• [x] Snapshot of UI changes, if any.

Reviewers

@mjkkirschner

FYI

@smangarole