Travis: add ubuntu xenial and bionic

[janlazo]

Ubuntu Trusty reached EOL since April 2019.

Close #176

[janlazo]

is $ENV hack to detect vim 8 still needed in test.vader? I don't know what it's suppose to do. Isn't v:version or has('patch-8') enough for version detection?

[junegunn]

I guess the purpose of the ENV hack was to test Ruby or Python installer even on Vim 8.

vim-plug/test/test.vader

Lines 21 to 23 in 226d6ab

	if $ENV != 'vim8'
	call add(patch, 'let s:vim8 = 0')
	endif

That was when the job feature of Vim 8 was new, unstable, and not widely available. Three years later, it doesn't make much sense to test Ruby/Python installers with Vim 8 anymore.

So basically we need to test:

1. with Vim 8 or later (nightly)
2. with Neovim
3. with Vim 7.x for Ruby installer
4. with Vim 7.x for Python installer
5. with Vim 7.x for Python3 installer

[junegunn]

Thanks!