Allow upgrading to the latest develop version if no new stable release using "platformio upgrade"

platformio/commands/upgrade.py

@@ -34,8 +34,17 @@ def cli():
         click.secho("Please wait while upgrading PlatformIO ...",
                     fg="yellow")
 
+        to_develop = False
+        try:
+            from pkg_resources import parse_version
+            to_develop = parse_version(last) < parse_version(__version__)
+        except ImportError:
+            pass
+
         cmds = (
-            ["pip", "install", "--upgrade", "platformio"],
+            ["pip", "install", "--upgrade",
+             "https://github.com/platformio/platformio/archive/develop.zip"
+             if to_develop else "platformio"],
             ["platformio", "--version"]
         )
 
@@ -53,10 +62,11 @@ def cli():
                     r = util.exec_command(cmd)
 
                 assert r['returncode'] == 0
-            assert last in r['out'].strip()
+            assert "version" in r['out']
+            actual_version = r['out'].strip().split("version", 1)[1].strip()
             click.secho(
-                "PlatformIO has been successfully upgraded to %s" % last,
-                fg="green")
+                "PlatformIO has been successfully upgraded to %s" %
+                actual_version, fg="green")
             click.echo("Release notes: ", nl=False)
             click.secho("http://docs.platformio.org/en/latest/history.html",
                         fg="cyan")