slugbuilder: Bump CURL_TIMEOUT to 10 minutes

CURL_TIMEOUT is the total time spent downloading a file in a buildpack.
The default in the Ruby buildpack is 30s, which will only work reliably
on servers with good connections to S3, this patch bumps it up to 10
minutes.

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
slugbuilder, receiver: Implement build caching

Build caching is implemented by downloading a cache tarball from
blobstore before starting the build, and uploading the cache directory
to blobstore after the build is complete.

Closes #619

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #653 from flynn/build-caching

Implement build caching