(maint) Update puppet version in Travis configuration

Without this patch we'll test against all Puppet 3.0 versions, but not
4.  This is a problem because the travis configuration should be
relatively future proof to minimize surprises.

This patch addresses the problem by using >= instead of ~> in the
dependency specification.  The patch also combines two different
notification sections into a single entry.

.travis.yml

@@ -1,15 +1,13 @@
 language: ruby
 bundler_args: --without development
 script: "bundle exec rake spec SPEC_OPTS='--format documentation'"
-notifications:
-  email: false
 rvm:
   - 1.9.3
   - 1.8.7
   - ruby-head
 env:
   - PUPPET_GEM_VERSION="~> 2.7"
-  - PUPPET_GEM_VERSION="~> 3"
+  - PUPPET_GEM_VERSION=">= 3.0.0"
 matrix:
   allow_failures:
     - rvm: ruby-head
@@ -19,6 +17,7 @@ matrix:
     - rvm: ruby-head
       env: PUPPET_GEM_VERSION="~> 2.7"
 notifications:
+  email: false
   webhooks:
     urls:
       - https://puppet-dev-community.herokuapp.com/event/travis-ci/