Remove travis-mono solution.

Instead use the Mono solution platform.

.travis.yml

@@ -10,10 +10,10 @@ before_install:
   - wget -O .nuget/nuget.exe https://dist.nuget.org/win-x86-commandline/latest/nuget.exe
 install:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt-get install -y gtk-sharp2 ; fi
-  - mono .nuget/nuget.exe restore Avalonia.travis-mono.sln
+  - mono .nuget/nuget.exe restore Avalonia.sln
   - mono .nuget/nuget.exe install xunit.runner.console -Version 2.1.0 -OutputDirectory testrunner
 script:
-  - xbuild /p:Configuration=Release Avalonia.travis-mono.sln
+  - xbuild /p:Platform=Mono /p:Configuration=Release Avalonia.sln
   - ./tests/run-tests.sh
 notifications:
   email: false