Bump System.Collections.Immutable from 1.6.0 to 1.7.0 (#132)

Bumps [System.Collections.Immutable](https://github.com/dotnet/corefx) from 1.6.0 to 1.7.0.
- [Release notes](https://github.com/dotnet/corefx/releases)
- [Commits](https://github.com/dotnet/corefx/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

src/Hocon.Immutable/Hocon.Immutable.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Collections.Immutable" Version="1.6.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="1.7.0" />
   </ItemGroup>
 
   <ItemGroup>