Remove redundant development dependency from gemspec

Since paperclip is defined as a runtime dependency, it does not need to
be defined as a development dependency.
use https rubygems source
remove Gemfile.lock
add Gemfile.lock back to .gitignore
use released paperclip gem
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Merge pull request #22 from teeparham/gemspec_paperclip

Remove redundant development dependency from gemspec
Merge pull request #23 from teeparham/https_rubygems

use https rubygems source
Merge pull request #24 from teeparham/gemfile

cleanup Gemfile
Add ruby 2.0, 2.1 to travis
Add .ruby-version
Update readme
Update travis badge links [ci skip]
Remove ruby 1.8.7, 1.9.2 from travis
Fix build for rubinius

* Only test latest rubinius
Update gemspec

* Remove development dependency on activerecord-jdbcsqlite3-adapter
that is conditional on RUBY_PLATFORM since it will not get built into
the gemspec properly. Platform-dependent dependencies are defined in
the Gemfile.
* Specify paperclip version dependency
* Specify versions for bundler & activerecord dependencies
* Add me as author
* Update descriptions
Change README file extension to .md [ci skip]
Convert readme to markdown [ci skip]
Specify minimum required ruby version of 1.9.3
Remove paperclip require; gem does that