Travis: Use dotnet core sdk

Full Framework version of FSAC is deprecated:
ionide/FsAutoComplete#506

.travis.yml

@@ -3,7 +3,7 @@ language: emacs-lisp
 # See https://github.com/travis-ci/travis-ci/issues/9061
 # and https://github.com/moby/moby/issues/22801
 sudo: required
-dist: trusty
+dist: bionic
 cache:
   - directories:
       # Cache stable Emacs binaries (saves 1min per job)
@@ -22,19 +22,21 @@ env:
   - EMACS_VERSION=snapshot
 
 before_install:
-  # Add mono repo
   - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-  - echo "deb http://download.mono-project.com/repo/ubuntu stable-trusty main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
-  - travis_retry sudo  apt-get -qq update
-  - travis_retry sudo  apt-get install -y mono-devel 
-  # Configure $PATH: Executables are installed to $HOME/bin
+  - wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
+  - sudo dpkg -i packages-microsoft-prod.deb
+  - sudo add-apt-repository universe
+  - sudo apt-get update
+  - sudo apt-get install apt-transport-https
+  - sudo apt-get update
+  - sudo apt-get install dotnet-sdk-2.2
   - export PATH="$HOME/bin:$PATH"
   # Download the makefile to emacs-travis.mk
   - wget 'https://raw.githubusercontent.com/flycheck/emacs-travis/master/emacs-travis.mk'
   # Install Emacs (according to $EMACS_VERSION) and Cask
   - make -f emacs-travis.mk install_emacs
   - make -f emacs-travis.mk install_cask
-  - travis_retry sudo apt-get install -y fsharp mono-xbuild
+  - travis_retry sudo apt-get install -y fsharp
 
 script:
   - make test