Pass a GEM_HOME/GEM_PATH environment variable to 'gem install'

Author: joankaradimov

jruby-gradle-base-plugin/src/main/groovy/com/github/jrubygradle/GemUtils.groovy

@@ -95,8 +95,10 @@ class GemUtils {
                 // jbundler and/or jar-dependencies will not attempt to invoke
                 // Maven on a gem's behalf to install a Java dependency that we
                 // should already have taken care of, see #79
-                environment 'JBUNDLE_SKIP', true
-                environment 'JARS_SKIP', true
+                environment JBUNDLE_SKIP : true,
+                            JARS_SKIP : true,
+                            GEM_HOME : destDir.absolutePath,
+                            GEM_PATH : destDir.absolutePath
                 main JRUBY_MAINCLASS
                 classpath jRubyClasspath
                 args '-S', GEM, 'install'