Update platform to 2.7.0-dev, bump Readme to 2.6.1 (#6774)

Author: d-a-v

README.md

@@ -3,7 +3,7 @@ Arduino core for ESP8266 WiFi chip
 
 # Quick links
 
-- [Latest release documentation](https://arduino-esp8266.readthedocs.io/en/2.6.0/)
+- [Latest release documentation](https://arduino-esp8266.readthedocs.io/en/2.6.1/)
 - [Current "git version" documentation](https://arduino-esp8266.readthedocs.io/en/latest/)
 - [Install git version](https://arduino-esp8266.readthedocs.io/en/latest/installing.html#using-git-version) ([sources](doc/installing.rst#using-git-version))
 
@@ -36,7 +36,7 @@ Starting with 1.6.4, Arduino allows installation of third-party platform package
 #### Latest release [![Latest release](https://img.shields.io/github/release/esp8266/Arduino.svg)](https://github.com/esp8266/Arduino/releases/latest/)
 Boards manager link: `https://arduino.esp8266.com/stable/package_esp8266com_index.json`
 
-Documentation: [https://arduino-esp8266.readthedocs.io/en/2.6.0/](https://arduino-esp8266.readthedocs.io/en/2.6.0/)
+Documentation: [https://arduino-esp8266.readthedocs.io/en/2.6.1/](https://arduino-esp8266.readthedocs.io/en/2.6.1/)
 
 ### Using git version
 [![Linux build status](https://travis-ci.org/esp8266/Arduino.svg)](https://travis-ci.org/esp8266/Arduino)

platform.txt

@@ -5,8 +5,8 @@
 # For more info:
 # https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification
 
-name=ESP8266 Boards (2.6.1)
-version=2.6.1
+name=ESP8266 Boards (2.7.0-dev)
+version=2.7.0-dev
 
 # These will be removed by the packager script when doing a JSON release
 runtime.tools.xtensa-lx106-elf-gcc.path={runtime.platform.path}/tools/xtensa-lx106-elf