Attempt to work around CI failures

After exploring the failures locally, it appears that term-ansicolor is the
culprit. Version 1.10.0 introduced an undocumented (as far as I can tell,
but its CHANGES file stops at 1.7.1 :-( ) dependency on ruby 2.5, by using
Hash#slice.

So use 1.9.0 you say! Except 1.9.0 doesn't work at all (it's trying to call
start_with? on a Symbol). I tried all the commits between 1.9.0 and 1.10.0
and all had some problem or another.

Luckily, the latest version needed is 1.3.x (by coveralls), so if I specify
that here, that's the one that's installed and bundle rake exec runs fine,
so hopefully this will fix the CI failures. We'll see...

Gemfile

@@ -45,5 +45,6 @@ group :test do
   gem 'request_store'
   gem 'rspec', '~> 3.1'
   gem 'rubocop', '~> 0.49.0'
+  gem 'term-ansicolor', '~> 1.3.0'
   gem 'yard'
 end