Update nuget packages to 2.x

Now that we're changing the calling conventions to cdecl, rev the major
version number so that it appears distinct from the continuous stream of
versions that were built with stdcall.

There's no real functional change here, but it clearly delineates any
maintenance updates to prior versions.

Author: ethomson

appveyor.yml

@@ -1,4 +1,4 @@
-version: 1.0.{build}
+version: 2.0.{build}
 
 branches:
   only:

nuget.package/NativeBinaries.nuspec

@@ -2,7 +2,7 @@
 <package xmlns="http://schemas.microsoft.com/packaging/2012/06/nuspec.xsd">
   <metadata>
     <id>LibGit2Sharp.NativeBinaries</id>
-    <version>1.0.155</version>
+    <version>2.0.0</version>
     <authors>LibGit2Sharp contributors</authors>
     <owners>nulltoken</owners>
     <licenseUrl>https://raw.githubusercontent.com/libgit2/libgit2/master/COPYING</licenseUrl>