Travis/AppVeyor compatibility

Without artifacts/deploying yet, I'm not sure about what the end result will be

.travis.yml

@@ -12,45 +12,22 @@ branches:
   only:
     - master
 
+# This is .NET Core project, we're not building with Mono
+mono: none
+
+# ASF requires .NET Core 2.0+
+# TODO: We should target stable 2.0.0 once it's released
+dotnet: 2.0.0-preview1-005977
+
 # This is our main build matrix
 matrix:
 # We can use fast finish, as we don't need to wait for all builds to mark it as failed/passed
   fast_finish: true
-  allow_failures:
-#   Mono is often broken, we might need to enable/disable allowing failures on as-needed basis
-#   Currently we allow all Mono builds to fail, as it has serious problems, such as random SIGABRTs
-#   This is decent because it allows us to track Mono builds and fix ASF if needed, while not going crazy over false alarms
-#   Ref: https://travis-ci.org/JustArchi/ArchiSteamFarm/jobs/192428388#L2007-L2009
-    - os: linux
-    - os: osx
   include:
-#   We're building ASF with Mono on latest versions of Linux and OS X
+#   We're building ASF with dotnet on latest versions of Linux and OS X
 #   Ref: https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments
     - os: linux
       dist: trusty
       sudo: false
-      mono: weekly
-    - os: linux
-      dist: trusty
-      sudo: false
-      mono: latest
     - os: osx
       osx_image: xcode8.3
-      mono: weekly
-    - os: osx
-      osx_image: xcode8.3
-      mono: latest
-
-# Initialize our envsetup before building, we're adding some extra environment properties there
-before_script:
-  - source mono_envsetup.sh
-
-# Push building notifications on Gitter
-notifications:
-  email: false
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/df82484f12510c3f2516
-    on_success: always  # options: [always|never|change] default: always
-    on_failure: always  # options: [always|never|change] default: always
-    on_start: never     # options: [always|never|change] default: always