pio: fix git builds & bump current platform

- platform == platform_latest
- mcspr/toolchain-xtensa via https://github.com/platformio/platformio-core/issues/3612

Yet, this does not avoid useless warnings that platform does
not specify platformio/* prefix in the package spec :(

It should be available in the next espressif8266 version:
0859336019

.travis.yml

@@ -29,8 +29,7 @@ jobs:
     - stage: Test Host
     - stage: Test WebUI
     - stage: Test PlatformIO Build
-      env: TEST_ENV=esp8266-4m-base
-    - env: >-
+      env: >-
         TEST_ENV=esp8266-4m-latest-base TEST_EXTRA_ARGS="-a
         test/build/extra/secure_client.h"
     - stage: Release

code/platformio.ini

@@ -31,12 +31,14 @@ extra_configs =
 #   arduino core 2.6.1 = espressif8266@2.3.0 (not supported)
 #   arduino core 2.6.2 = espressif8266@2.3.1 (not supported)
 #   arduino core 2.6.3 = espressif8266@2.4.0 (not supported)
-#   arduino core 2.7.1 = espressif8266@2.5.3 (platform)
-#   arduino core 2.7.4 = espressif8266@2.6.2 (platform_latest)
+#   arduino core 2.7.1 = espressif8266@2.5.3
+#   arduino core 2.7.2 = espressif8266@2.6.0
+#   arduino core 2.7.3 = espressif8266@2.6.1
+#   arduino core 2.7.4 = espressif8266@2.6.2 (platform)
 #
 # ------------------------------------------------------------------------------
 
-platform = espressif8266@2.5.3
+platform = espressif8266@2.6.2
 platform_latest = espressif8266@2.6.2
 
 # ------------------------------------------------------------------------------
@@ -120,10 +122,9 @@ debug_flags =
 
 shared_libdeps_dir = libraries/
 
-# TODO: right now we depend on external toolchain url b/c toolchain version is hard-coded into the platform
 git_platform_packages =
     framework-arduinoespressif8266 @ https://github.com/esp8266/Arduino.git
-    toolchain-xtensa @ ~2.100100.200706
+    mcspr/toolchain-xtensa @ 5.100200.200918
 
 lib_deps =
     https://github.com/me-no-dev/ESPAsyncTCP.git#7e9ed22