use PLATFORM=jruby instead of JRUBY=true

dist/jruby.rake

@@ -1,5 +1,5 @@
 file pkg("foreman-#{version}-jruby.gem") => distribution_files do |t|
-  sh "env JRUBY=true gem build foreman.gemspec"
+  sh "env PLATFORM=jruby gem build foreman.gemspec"
   sh "mv foreman-#{version}-java.gem #{t.name}"
 end
 

foreman.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'term-ansicolor', '~> 1.0.7'
   gem.add_dependency 'thor',           '>= 0.13.6'
 
-  if ENV["JRUBY"]
+  if ENV["PLATFORM"] == "jruby"
     gem.add_dependency "posix-spawn", "~> 0.3.6"
     gem.platform = Gem::Platform.new("java")
   end