Installs Roslyn 0.7.0.0

Everything works fine after calling
Install-Package Microsoft.CodeAnalysis -Pre
in package manager.

It turns out that the problem I was experiencing was related to the fact
that I had downgraded from Roslyn 0.7.0.0 to Roslyn 0.6.4033103 and
there was a bit of a mess with versions of various packages.