Fetch libicu via Travis, switch back to container-based

This is a test.  I don't understand the details of how the `bundle config
build.charlock_holmes` bit works/fails, so the intent here is to get a PR posted
for purposes of getting feedback via the full Travis machinery.

.travis.yml

@@ -1,3 +1,10 @@
+language: ruby
+sudo: false
+addons:
+  apt:
+    packages:
+    - libicu-dev
+    - libicu48
 before_install: script/travis/before_install
 rvm:
   - 2.0.0

script/travis/before_install

@@ -5,18 +5,6 @@ set -ex
 # Fetch all commits/refs needed to run our tests.
 git fetch origin master:master v2.0.0:v2.0.0 test/attributes:test/attributes test/master:test/master
 
-sudo apt-get update
-
-script/vendor-deb libicu48 libicu-dev
-if ruby -e 'exit RUBY_VERSION >= "2.0" && RUBY_VERSION < "2.1"'; then
-  # Workaround for https://bugs.ruby-lang.org/issues/8074. We can't use this
-  # solution on all versions of Ruby due to
-  # https://github.com/bundler/bundler/pull/3338.
-  bundle config build.charlock_holmes --with-icu-include=$(pwd)/vendor/debs/include --with-icu-lib=$(pwd)/vendor/debs/lib
-else
-  bundle config build.charlock_holmes --with-icu-dir=$(pwd)/vendor/debs
-fi
-
 # Replace SSH links to submodules by HTTPS links.
 sed -i 's/[email protected]:/https:\/\/github.com\//' .gitmodules