Modify Ruby versions for Travis

Change the .travis.yml file so that Travis no longer runs
the tests against Ruby 1.8.7 and 1.9.2 since the Endian
modifiers were not added to Array#pack until 1.9.3.  While
we're add it add Ruby 2.0.0 and 2.1.0 to the file to run
the tests against the newer Rubies.
Add style changes per @Fryguy

1. Remove jruby & rubinius 1.8 statements from .travis.yml
2. Undo attempts to fix Rubocop "void context" warnings which
result in redundant code in tests.
3. Simplify validate_definition_entry_modifier & rename to
valid_definition_entry_modifier? since it doesn't raise an exception
4. Simplify get_size
5. Align assignments in constant definitions when possible.
6. Remove unnecessary "()" in new lambda syntax.