Rename cont_t for Core 2.4.2 and higher

[mcspr]

88e2ecc#diff-bd6de869be74ac5eaf90f23e4b75b857 added extern g_cont:

espurna/code/espurna/utils.ino

Lines 9 to 12 in 88e2ecc

	extern "C" {
	#include <cont.h>
	extern cont_t g_cont;
	}

but it is no longer defined in 'master' of esp8266/Arduino, now it is cont_t* g_pcont:
esp8266/Arduino@5d5ea92#diff-3fa12668b289ccb95b7ab334833a4ba8L35

This PR:

• added define check for core version when using g_cont/g_pcont
• updated travis envs to use newest platform, so this is tested

[mcspr]

rebased.
platform = espressif8266@>=1.8.0 was not working as expected. newer platform must be installed manually.

[mcspr]

esp8266/Arduino issue about this esp8266/Arduino#5118
if pr linked to it comes with 2.5.0 - ifdef here will be g_cont 2.3.0, 2.4.0, 2.4.1, g_pcont 2.4.2 and built-in ESP class method for latter

[Misiu]

@mcspr what about ESP.getFreeContStack() mentioned here.
PR containing this function is already merged.

[mcspr]

@Misiu yep, you are right. Right when I wrote my comment, pr was merged and I missed notification.
I'll open another one with new method

[Misiu]

@mcspr that was fast 😄