replace Makefile with equivalent Rakefile

Signed-off-by: Gwenhael Le Moine <gwenhael.le.moine@gmail.com>

Makefile

@@ -1,8 +0,0 @@
-gem: rpl.gemspec
-	gem build rpl.gemspec
-
-run:
-	ruby -Ilib bin/rpl
-
-test:
-	find ./spec/ -name \*.rb -exec ruby -Ilib {} \;

Rakefile

@@ -0,0 +1,13 @@
+require 'rake/testtask'
+
+Rake::TestTask.new do |t|
+  t.pattern = "spec/*_spec.rb"
+end
+
+task :gem do
+  sh "gem build rpl.gemspec"
+end
+
+task :run do
+  sh "ruby -Ilib bin/rpl"
+end