Correct upload speed for m5stick-c board (#240)

The current upload speed gives an error when trying to upload on m5stick-c:
```
A fatal error occurred: Timed out waiting for packet header
```
With 1500000 the upload works correctly

Author: 1technophile

boards/m5stick-c.json

@@ -25,7 +25,7 @@
     "maximum_ram_size": 327680,
     "maximum_size": 4194304,
     "require_upload_port": true,
-    "speed": 460800
+    "speed": 1500000
   },
   "url": "http://www.m5stack.com",
   "vendor": "M5Stack"