Improve rake tasks for specs

- clean coverage directory before running specs to avoid simplecov
  slowdowns
- add valgrind task to run valgrind; normal spec does not run it

spec/propane_spec.rb

@@ -119,7 +119,7 @@ EOF
     results = Results.new(stdout, stderr, status)
     # Valgrind is only reliably available on Linux, so limit the leak check to
     # Linux platforms.
-    if RUBY_PLATFORM =~ /linux/
+    if RUBY_PLATFORM =~ /linux/ && ENV["spec-valgrind"]
       stdout, stderr, status = Open3.capture3("valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --verbose spec/run/testparser")
       vgout = stdout + stderr
       File.binwrite("spec/run/.vgout", vgout)