Try using github syntax for GxEPD2 -- travis build was failing w/ version syntax

Author: sidoh

platformio.ini

@@ -31,13 +31,7 @@ lib_deps_external =
   ; and the webserver builtin to the espressif SDKs define.
   WiFiManager=https://github.com/sidoh/WiFiManager#async_support
 
-  ; Using a forked version because maintainer does not wish to add a partial
-  ; update method which also flushes the whole buffer, as some modules don't
-  ; support this.
-  ;
-  ; Context: https://github.com/ZinggJM/GxEPD2/pull/18
-  ; sidoh/GxEPD2#partial_display
-  GxEPD2@1.2.3
+  GxEPD2=https://github.com/ZinggJM/GxEPD2#1.2.3
 extra_scripts =
   pre:scripts/platformio/build_web.py
   post:scripts/platformio/build_full_image.py