Gemfile: add ruby RUBY_VERSION version hint

[olleolleolle]

• this helps bundler choose the right versions of gems for older Rubies.

(Until Bundler can figure all of this out on its own, we can regenerate the lockfile on each build.)

[olleolleolle]

• This makes the 1.9.3 build work
• The 1.8.7 build can never work since its URI module does not have #encode_www_form
• ree is also 1.8.7 - ree-1.8.7-2012.02 and fails for the same reason

Error: test_upgrade_mozillastore(TestHTTPCookieJar::WithMozillaStore)
  NoMethodError: undefined method `encode_www_form' for URI:Module
/home/travis/build/sparklemotion/http-cookie/lib/http/cookie_jar/mozilla_store.rb:303:in `upgrade_database'

[knu]

It was an accident that 1.8 compatibility was broken, http-cookie still supports 1.8.

[olleolleolle]

@knu Cool!

• rebased
• added back ree, 1.8.7
• added the RUBY_VERSION marker to Gemfile
• added exact versions to JRuby Travis builds

[olleolleolle]

Perhaps we can talk about whether to keep rbx-2 in the matrix, instead?

Example build result

Requested binary installation but no rubies are available to download, consider skipping --binary flag.
Gemset '' does not exist, 'rvm rbx-2 do rvm gemset create ' first, or append '--create'.

[knu]

@olleolleolle I think we can drop support for rbx-2.

[knu]

Do we need this change? I don't like to hard-code minor versions. For jruby-9.1.x you could just say jruby, if jruby-9 pointed to jruby-9.0.x.

[knu]

Maybe I could just merge this part, since that's what this PR is about.

[olleolleolle]

I removed the other changes; would this make it mergeable?