Keep existing environment variables when spawning 'gem install'

Author: joankaradimov

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

@@ -95,7 +95,8 @@ 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
-                setEnvironment 'JBUNDLE_SKIP' : true, 'JARS_SKIP' : true
+                environment 'JBUNDLE_SKIP', true
+                environment 'JARS_SKIP', true
                 main JRUBY_MAINCLASS
                 classpath jRubyClasspath
                 args '-S', GEM, 'install'