corrected command line argument "init"

The previous value "istall" seems like a mistake.
Problem described here https://github.com/gitextensions/gitextensions/issues/2564
Merge pull request #2565 from Mellbourn/master

corrected command line argument "init"
attempt on using std Nuget restore and xbuild
NuGet Version: 2.8.50506.491
removing Nuget.sh as it is not needed anymore
apt-get install mono-complete
Merge pull request #2567 from fraga/travisBuild

Travis build, towards #2561