Clone a single revision

Previously, when a buildpack was cloned, the entire buildpack
repository was fetched.  This resulted in a slower clone than was
otherwise necessary.  This change executes the clone with a --depth 1
which only fetches a single revision and no history.  This results in
noticeable speed gains.

Buildpack                       Full    Depth 1
===============================================
cloudfoundry-buildpack-java:    2.031s  1.451s
heroku-buildpack-nodejs:        6.840s  1.577s
heroku-buildpack-ruby:          5.075s  1.785s

[#50080879]
Initialize buildpack submodules

Previously, when a buildpack was cloned, any submodules containted
within that repository were not initialized.  This means that any
split of buildpack code across repositories was difficult if not
impossible.  This change adds --recurse-submodules to the clone
command to initialize any declared submodules.

[#50081011]
Merge pull request #22 from nebhale/50080879-depth-1

Clone a single revision
Merge pull request #23 from nebhale/50081011-submodules

Initialize buildpack submodules
loosen up .ruby-version (don't specify patchlevel)
Loosen em timeout requirements for travis
DEA won't start app instances it doesn't have the capacity for

[fixes #50202171]