Fix hostname support for Enterprise installations of GitHub (#118)

Per the [documentation](https://developer.github.com/enterprise/2.16/v3/#current-version), enterprise installations support a base URL of  `http(s)://[hostname]/api/v3` in addition to the existing support of `http(s)://api.[hostname]/`.  This PR updates the module to _always_ use the `http(s)://[hostname]/api/v3` syntax when `[hostname]` is _not_ `github.com`.

Fixes #117

Author: v2kiran

GitHubCore.ps1

@@ -153,7 +153,14 @@ function Invoke-GHRestMethod
 
     $hostName = $(Get-GitHubConfiguration -Name "ApiHostName")
 
-    $url = "https://api.$hostName/$UriFragment"
+    if ($hostName -eq 'github.com')
+    {
+        $url = "https://api.$hostName/$UriFragment"
+    }
+    else
+    {
+        $url = "https://$hostName/api/v3/$UriFragment"
+    }
 
     # It's possible that we are directly calling the "nextLink" from a previous command which
     # provides the full URI.  If that's the case, we'll just use exactly what was provided to us.
@@ -966,4 +973,3 @@ function Get-MediaAcceptHeader
 
     return ($acceptHeaders -join ',')
 }
-