Increase curl timeout to two minutes

In #165, a few users are getting time outs.

```
  Compiling Ruby
 !
    ERROR: Command: 'set -o pipefail; curl --fail --retry 3 --retry-delay 1 --connect-timeout 3 --max-time 20 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/ruby-1.9.3-jruby-1.7.3.tgz -s -o - | tar zxf -' failed unexpectedly:
    ERROR: 
    ERROR: gzip: stdin: invalid compressed data--format violated
    ERROR: tar: Unexpected EOF in archive
    ERROR: tar: Unexpected EOF in archive
    ERROR: tar: Error is not recoverable: exiting now
 !
```
make curl max_timeout variable with env.
[ci skip] changelog
don't require rake to be present for Ruby 1.9+
Merge pull request #175 from icco/patch-1

Allow curl timeout to be configurable
ruby 1.9 also has rake in stdlib
add rake detect tests
un-focus test
Merge pull request #190 from heroku/schneems/rake-version

don't require rake to be present for Ruby 1.9+
Hide default bundler clean output
Fix LPXC deadlock issue 

ryandotsmith/lpxc#7
Merge pull request #174 from heroku/schneems/less-verbose-default-cache

Hide default bundler clean output
remove rake tasks that use vulcan

the buildpack uses [hammer](http://github.com/hone/hammer)
Merge pull request #192 from heroku/schneems/fix-lpxc

Fix LPXC deadlock issue
Merge pull request #193 from heroku/remove_vulcan

remove rake tasks that use vulcan
Bundler 1.4.0.pre.1, set parallel install to 2
git fixes sin Bundler 1.4.0.pre.2
need to invalidate the CloudFront cache first, default to S3 for now
instrument bundler to stdout
try -j4