Update .travis.yml

Installation of NUnit.Runners got lost in previous commit

Author: FObermaier

.travis.yml

@@ -25,5 +25,6 @@ install:
   - curl -sSL https://raw.githubusercontent.com/dotnet/cli/rel/1.0.0-preview2/scripts/obtain/dotnet-install.sh | bash /dev/stdin --version "$CLI_VERSION" --install-dir "$DOTNET_INSTALL_DIR"
   - export PATH="$DOTNET_INSTALL_DIR:$PATH"  
   - nuget restore GeoAPI.sln
+  - nuget install NUnit.Runners -version 3.6.0 -OutputDirectory .testRunner
 script:
   - ./build.sh