Avoid broken net-http-persistent v2.9.2/3

Broken by drbrain/net-http-persistent#56
Allow jruby-XYmode failures on Travis due to RVM bug

travis-ci/travis-ci#1948
Bump net-http-persistent

Needed fix for drbrain/net-http-persistent#59
Cache Bundler dependencies on Travis CI

- Even after the cache is restored, `bundle install` still needs to hit
  RubyGems.org because there is no Gemfile.lock. There doesn't seem to
  be a way to tell it that everything it needs is right there in the
  `./bundle` directory.

- Can't use `--standalone` due to bundler/bundler#2851

- It's not necessary to manually update RubyGems and Bundler anymore.
Handle Patron "Connection time-out" properly
Adds a retry_if option to the retry middleware

retry_if allows users to provide a block that
will receive the env and exception objects
and should decide if the call should be retried
or not.

Fixes #298.
Retry only if the method is idempotent

Only run retries if the method is known to be
idempotent. If it isn't, switch to use the retry_if
block to make the final decision, if it is known to
be idempotent allow it to be retried by default.
Merge pull request #340 from mauricio/issue-298

Retry only HTTP methods known to be idempotent

User can optionally configure other methods.

Fixes #298
Add HTTPClient to list of adapters in README
Merge pull request #342 from jaimeiniesta/patch-1

Add HTTPClient to list of adapters in README
Only try to cache dependencies when Amazon secret is available

Fixes the case when someone forks the repo and runs Travis on their own.
Also, allows the use-case of restoring dependencies only without
necessarily trying to cache them.
Update Travis settings for Rubinius

/cc @brixen
Add full url for get /nigiri in README
Merge pull request #350 from chrishunt/update-readme

Add full url for get /nigiri in README
Merge remote-tracking branch 'lostisland/master' into add_ciphers_to_net_http

* lostisland/master:
  Add full url for get /nigiri in README
  Update Travis settings for Rubinius
  Only try to cache dependencies when Amazon secret is available
  Add HTTPClient to list of adapters in README
  Retry only if the method is idempotent
  Adds a retry_if option to the retry middleware
  Handle Patron "Connection time-out" properly
  Cache Bundler dependencies on Travis CI
  Bump net-http-persistent
  Allow jruby-XYmode failures on Travis due to RVM bug
  Avoid broken net-http-persistent v2.9.2/3