Updates the enum to accept -ve values

When using enum values, a value of a -ve number is not allowed because
it is implemented as an array. Changes the implementation to a hash to
accept -ve numbers
Do not reopen class bodies if no fields defined
Preserve underscores in Constantize function

Fixes #56.
Add before_install task to travis for libzmq
Only build for 1.9.2 and 1.9.3 for now

I need to take the time to get specs passing for jruby and ruby head.
Merge pull request #55 from hassox/master

Update enum to accept negative values
Fix deprecation env specs
Merge pull request #56 from localshred/56_underscored_message_names

Preserve underscores in Constantize function
Merge branch 'master' of github.com:localshred/protobuf
Bump version 2.6.4
Allow setting protoc source as an environment variable when compiling
Merge pull request #57 from hassox/protoc-src

Allow setting protoc source as an environment variable when compiling
Add Install section, PROTOC_SRC env on gem install

Extended cleanup of the README formatting and
clarifying/removing some sections.