Add explicit netstandard2.0 support

[am11]

Although netstandard1.x libs are compatible with higher version of
standards, it is a good idea to target each standard specifically
if the dependencies are different. In case of NETSTANDARD 2.0,
we don't need additional dependencies other than the default
NETStandard.Library which is a
flat package.

Also removed the mono-specific solution and project files, as the
latest Mono uses CSC (C# Roslyn compiler from dotnet/roslyn repo),
instead of MCS and MSBuild (from microsoft/MSBuild repo) instead of
xbuild. See http://www.mono-project.com/docs/about-mono/releases/5.0.0
for details.

[am11]

TraivsCI seems to be disabled atm for this repo: https://travis-ci.org/maxmind/MaxMind-DB-Reader-dotnet.

[oschwald]

Thanks! This looks great as always. I had one question about the AppVeyor config. Travis should be re-enabled now; I am not sure what happened there.

Sorry for the delay in review; I was on holiday.

[oschwald]

Is there a reason the debug test build for 1.4 was removed?

[am11]

I was thinking if it makes sense to only configure the build+release jobs for the latest framework and release for older ones.

[am11]

TravisCI seems to still have some issue here https://travis-ci.org/maxmind/MaxMind-DB-Reader-dotnet/branches, but working against my fork https://travis-ci.org/am11/MaxMind-DB-Reader-dotnet.

Maybe in repo settings there, the PR trigger is off:

[oschwald]

Weird. "Build pull request updates" was enabled. I deleted the integration from GitHub and then re-enabled it again in Travis. Hopefully it should work now. Could you try pushing something?

[am11]

Yay, it worked! 🌮

[am11]

@oschwald, with dotnet cli 2.0, if we have full mono or only reference assemblies of some desktop TFM (say net45, net32, net20) installed on Unix, then we can use dotnet restore, build, test commands for those TFMs on Unix as well. Tried it here: https://github.com/neris/NGettext/blob/a701ac6/.travis.yml. But in our matrix, we are not installing dotnet-cli in mono jobs that's why we cant use dotnet commands which makes sense as well.

[oschwald]

Thanks!