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.
Merge pull request #175 from icco/patch-1

Allow curl timeout to be configurable