Add install step

Author: asbjornu

.travis.yml

@@ -5,6 +5,8 @@ env:
     - DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
     - DOTNET_CLI_TELEMETRY_OPTOUT=1
 dotnet: 2.1.401
+install:
+  - dotnet restore
 script: dotnet test -c Release /p:CollectCoverage=true /p:CoverletOutputFormat=opencover /p:Exclude="[JsonLD.Test*]*"
 after_success:
   - curl -s https://codecov.io/bash > codecov