.travis.yml in global_session-3.2.3 vs .travis.yml in global_session-3.2.4

@@ -1,8 +1,11 @@
 language: ruby
 cache: bundler
 rvm:
   - 1.8.7
-  - 2.1
+  - 2.1.8
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get install -y libgmp3-dev
 script:
   - bundle exec rake ci:spec
 bundler_args: --without development