DNX build failure fixes

- File.IO needed to be referenced now that we have an API around reading
  a license file from disk

- ifdef reading a license location from env variables in dotnetcore

Author: Mpdreamz

src/Nest/project.json

@@ -108,6 +108,7 @@
       },
       "dependencies": {
         "System.Collections": "4.0.11-beta-23225",
+        "System.IO.FileSystem": "4.0.1-beta-23225",
         "System.Diagnostics.Tools": "4.0.1-beta-23225",
         "System.Dynamic.Runtime": "4.0.11-beta-23516",
         "System.Linq": "4.0.0-beta-23109",

src/Tests/Framework/Integration/Process/ElasticsearchNode.cs

@@ -255,7 +255,11 @@ private void ValidateLicense()
 			if (license.IsValid && license.License.Status == LicenseStatus.Active) return;
 
 			var exceptionMessageStart = "Server has license plugin installed, ";
+#if DOTNETCORE
+			var licenseFile = string.Empty;
+#else
 			var licenseFile = Environment.GetEnvironmentVariable("ES_LICENSE_FILE", EnvironmentVariableTarget.Machine);
+#endif
 			if (!string.IsNullOrWhiteSpace(licenseFile))
 			{
 				var putLicense = client.PostLicense(new PostLicenseRequest { License = License.LoadFromDisk(licenseFile) });