Wire the internal GemInstaller implementation into the public API

src/main/groovy/com/github/jrubygradle/groovygem/GemInstaller.groovy

@@ -2,6 +2,8 @@ package com.github.jrubygradle.groovygem
 
 import groovy.transform.CompileStatic
 
+import com.github.jrubygradle.groovygem.internal.GemInstaller as GemInstallerImpl
+
 /**
  * GemInstaller manages the installation of a .gem file into a given directory
  */
@@ -13,8 +15,8 @@ class GemInstaller {
         FAIL
     }
 
-    protected File installDirectory
-    protected List<File> gems
+
+    protected GemInstallerImpl impl
 
     GemInstaller(String installDir, String gemPath) {
         this(installDir, new File(gemPath))
@@ -25,7 +27,10 @@ class GemInstaller {
     }
 
     GemInstaller(String installDir, List<File> gemPaths) {
-        this.installDirectory = new File(installDir)
-        this.gems = gemPaths
+        impl = new GemInstallerImpl(installDir, gemPaths)
+    }
+
+    void install() {
+        impl.install()
     }
 }