Do not run tests against Node v8 on Travis

[jgerigmeyer]

Fixes #69

@mirisuzanne I also upgraded Herman, but the switch to Dart-Sass (at least I think that's it) is causing errors on two @example annotations:

• _register.scss:

» [WARNING] Error compiling @example scss:
get-function("darken") isn't a valid CSS value.
  ╷
6 │   /*! - #{$name}: #{$value} */
  │                     ^^^^^^
  ╵
  stdin 6:21  root stylesheet
@import 'tools';
@include register-function('darken', 'shade');

/*! Registered Functions… */
@each $name, $value in $functions {
  /*! - #{$name}: #{$value} */
}

• _themes.scss

» [WARNING] Error compiling @example scss:
("shade": 40%) isn't a valid CSS value.
   ╷
12 │ /* #{$name}: #{$value} */
   │                ^^^^^^
   ╵
  stdin 12:16  root stylesheet
@import 'tools';
$user-colors: (
  'primary': blue,
  'accent': red,
);
@include add-colors($user-colors);

$theme-colors: shades-of($user-colors, 2);
@include add-colors($theme-colors);

@each $name, $value in $theme-colors {
/* #{$name}: #{$value} */
}

If you want to fix those that's great; if not I can back out the dep upgrades.