update .NET SDK, use non-sudo Travis

.travis.yml

@@ -1,15 +1,13 @@
 dist: trusty
 sudo: false
 language: csharp
+dotnet: 2.1.300
 mono: none
 solution: FilterLists.sln
-before_install:
-- curl https://packages.microsoft.com/keys/microsoft.asc | gpg --dearmor > microsoft.gpg
-- sudo mv microsoft.gpg /etc/apt/trusted.gpg.d/microsoft.gpg
-- sudo sh -c 'echo "deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-ubuntu-trusty-prod trusty main" > /etc/apt/sources.list.d/dotnetdev.list'
-- sudo apt-get update
-- sudo apt-get install dotnet-sdk-2.1.200
-- sudo apt-get install sshpass
+addons:
+  apt:
+    packages:
+    - sshpass
 script:
 - scripts/build.sh
 env: