Fix travis build error for snap install

[hlovdal]

The latest travis build fails with the following error, https://travis-ci.org/github/cake-build/resources/jobs/741243741:

$ sudo snap install powershell --classic error: cannot perform the following tasks:

• Mount snap "powershell" (137) (snap "powershell" assumes unsupported features: command-chain (try to update snapd and refresh the core snap)) The command "sudo snap install powershell --classic" failed and exited with 1 during . Your build has been stopped.

According to https://forum.snapcraft.io/t/installing-snap-on-debian/6742:

Note: some snaps require new snapd features and will show an error such as snap "lxd" assumes unsupported features" during install. You can solve this issue by making sure the core snap is installed (snap install core) and it’s the latest version (snap refresh core).

and the error message itself also hints to this.

[gep13]

@hlovdal thank you again for fixing this up! This was on my list to investigate, so I really happy to see that this has been solved!

[hlovdal]

@gep13 Well, you still have some investigation to do because the build is still failing although now it pretends to be successful, #93.