Avalonia.Mono.sln -> Avalonis.travis-mono.sln

People were getting confused thinking the sln was for building on mono -
it's not, it's just for travis.

.travis.yml

@@ -4,7 +4,7 @@ os:
   - osx
 mono:
   - latest
-solution: Avalonia.mono.sln
+solution: Avalonia.travis-mono.sln
 before_install:
   - mkdir -p .nuget
   - wget -O .nuget/nuget.exe https://dist.nuget.org/win-x86-commandline/latest/nuget.exe