Update .travis.yml

This commit makes the following changes to the test matrix:

- Runs tests on ruby 2.1.5 instead of 2.0.0
- Runs tests on ruby 1.8.7 with puppet 3.x
- Adds an environment to run on an intermediate 3.x puppet version
  instead of latest (specifically 3.4.x)
- Adds an environment to run with the future parser on latest puppet.
  This would affect the test runs for the validate and spec checks.
- Runs the tests on docker

.travis.yml

@@ -1,13 +1,24 @@
 ---
+sudo: false
 language: ruby
 bundler_args: --without system_tests
 script: "bundle exec rake validate && bundle exec rake lint && bundle exec rake spec SPEC_OPTS='--format documentation'"
 matrix:
   fast_finish: true
   include:
   - rvm: 1.9.3
+    env: PUPPET_GEM_VERSION="~> 3.4.0"
+  - rvm: 1.8.7
     env: PUPPET_GEM_VERSION="~> 3.0"
-  - rvm: 2.0.0
+  - rvm: 1.9.3
+    env: PUPPET_GEM_VERSION="~> 3.0"
+  - rvm: 1.9.3
+    env: PUPPET_GEM_VERSION="~> 3.0" FUTURE_PARSER="yes"
+  - rvm: 2.1.5
     env: PUPPET_GEM_VERSION="~> 3.0"
+  - rvm: 2.1.5
+    env: PUPPET_GEM_VERSION="~> 3.4.0"
+  - rvm: 2.1.5
+    env: PUPPET_GEM_VERSION="~> 3.0" FUTURE_PARSER="yes"
 notifications:
   email: false