Pin minitest to v5.11.3 for Ruby v1.8.7 to fix build

This is because minitest > v5.11.3 uses look-behind assertions in regular
expressions which are not supported in the regular expression engine for
Ruby < v1.9 [1]. This has been causing build errors like this one [2].

As suggested by @nitishr in this comment [3], this commit pins minitest to
v5.11.3 in the integration test build for Ruby v1.8.7 which should hopefully fix
the build.

It's worth noting that minitest has also officially dropped support for Ruby <
v2.2 [4], but currently this doesn't appear to be affecting the build. I'm happy
to address this only if/when it actually becomes a problem.

[1]: minitest/minitest#803 (comment)
[2]: https://travis-ci.org/floehopper/metaclass/jobs/588418080
[3]: #370 (comment)
[4]: https://github.com/seattlerb/minitest/blob/15ed8e4ce504c8313058a1d6fc4918299be34328/History.rdoc#5122--2019-09-28

.travis.yml

@@ -73,7 +73,7 @@ matrix:
       gemfile: Gemfile
       env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
     - rvm: 1.8.7-p371
-      gemfile: gemfiles/Gemfile.minitest.latest
+      gemfile: gemfiles/Gemfile.minitest.5.11.3
       env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
     - rvm: 1.8.7-p371
       gemfile: gemfiles/Gemfile.minitest.2.11.2

gemfiles/Gemfile.minitest.5.11.3

@@ -0,0 +1,7 @@
+source 'https://rubygems.org'
+
+gemspec :path=>"../"
+
+group :development do
+  gem "minitest", "5.11.3"
+end