Removed NUnit.Runners as Travis should install it

Author: asbjornu

src/GitVersionCore.Tests/packages.config

@@ -7,7 +7,6 @@
   <package id="ModuleInit.Fody" version="1.5.8.0" targetFramework="net45" developmentDependency="true" />
   <package id="NSubstitute" version="1.9.2.0" targetFramework="net45" />
   <package id="NUnit" version="2.6.4" targetFramework="net45" />
-  <package id="NUnit.Runners" version="2.6.4" targetFramework="net45" />
   <package id="NUnitTestAdapter" version="2.0.0" targetFramework="net45" />
   <package id="Shouldly" version="2.7.0-beta0002" targetFramework="net45" />
   <package id="TestStack.ConventionTests" version="3.0.0-beta0002" targetFramework="net45" />