Add missing class declaration

README.markdown was missing details about declaring 'my_fw::pre'
and 'my_fw::post' which caused folks following the example to see
the following error:

Could not find dependency Class[My_w::Pre] for Firewall[BLAH]
Fix tests for Ruby 1.9.3 from 3e13bf3

Changes in 3e13bf3 broke tests for Ruby 1.9.3 which doesn't support
Enumerable on Strings. Workaround this by casting everything as an array and
flattening to prevent existing arrays from being encapsulated.
Merge pull request #109 from dcarley/fix_tests_ruby_193

Fix tests for Ruby 1.9.3 from 3e13bf3
Merge pull request #96 from sfozz/documentation-fixes

Add missing class declaration
Add Puppet 3.1 to travis-ci test matrix

Signed-off-by: Ken Barber <ken@bob.sh>