travis: add ubuntu bionic,xenial (#867)

Travis defaults to it since this year.
Ubuntu trusty reached EOL since April 2019.
Ubuntu bionic provides Vim 8.

Simplify OS install with Travis' apt addon
so that only Vim nightly will be compiled from source.

Ubuntu 14.04.6 provides ruby v.1.9.3, not 1.8.x or older.
Use it to test the ruby provider for backward compatibility.

Signed-off-by: Jan Edmund Lazo <jan.lazo@mail.utoronto.ca>

test/test.vader

@@ -18,10 +18,6 @@ Execute (Initialize test environment):
   \ ['function! ResetPlug()', 'let s:loaded = {}', 'endfunction',
   \  'function! CompareURI(a, b)', 'return s:compare_git_uri(a:a, a:b)', 'endfunction']
 
-  if $ENV != 'vim8'
-    call add(patch, 'let s:vim8 = 0')
-  endif
-
   call writefile(extend(readfile($PLUG_TMP), patch), $PLUG_TMP)
 
   set t_Co=256