Upgrade to support .NET Core

[jsancho]

Hi Brady, Seeing that you were open to take a PR for a .NET Core framework upgrade, I thought I would oblige :)

This has been really straightforward, it's mostly been a matter of upgrading to the new VS 2017 .csproj format and adjusting the targetting for net45, netcoreapp2 frameworks. That means that you will not be able to open the project from VS 2015, but I'd say that it's worth doing the jump after seeing how much of the verbose XML disappears.

• Note that rather than using netstandard I have targeted netcoreapp as that is a requirement of Microsoft.SqlServer.SqlManagementObjects

The project will now generate two separate builds for both frameworks, so you should be able to keep supporting .NET 4.5 once you publish the library. Matter of fact is that I have not touched the console project that you have for testing, and it seems to be accepting the new build just fine.

The only caveat is that without unit tests nor the "mdf" file that you use in Program.cs I don't have an easy way to test this in full. Nevertheless, I can confirm that it builds, runs, and that I could attach a debugger without issues.

The one massively obvious recommendation would be to ensure that you fully test it with an actual database. I'm also completely new to DbUp, so I don't have any existing migrations to use.

Lastly, thanks for creating this project, I'm really keen to get going with it!

Regards,

Jaume

Fixes #32

[yegor-n-a]

@bradymholt Any concerns / updates regarding this pull request so far?

[bradymholt]

@yegor-n-a @crazycrank @simonpinn @efranzinelli @thechups @robbaman @mwhitty - Would one of you be willing to vet and test this PR?

[yegor-n-a]

@bradymholt At the moment, the project includes neither unit nor integration tests. Did you mean, smoke testing or something much more substantial?

[bradymholt]

@yegor-n-a - Yes, smoke test and manual testing.