Update nuget.exe to v2.7 for servicestack builds

Servicestack doesn't build on mono with nuget 2.6 due to issues with the
package restore feature.

Nuget 2.7 enables package restore by default and servicestack now builds
without issue on mono/linux.
Merge pull request #449 from kppullin/servicestack_nuget_2_7

Update nuget.exe to v2.7 for servicestack builds