Read ruby-build revision from git if available

But don't assume that RUBY_BUILD_ROOT is where ruby-build's own files
reside, since RUBY_BUILD_ROOT can be overriden with alternate definition
files location.
Faster clone of Bats in CI

Avoid cloning the whole project's history which we'll never need
Merge pull request #612 from sstephenson/git-revision

Read ruby-build revision from git if available
add jruby 1.7.14 build
Merge pull request #620 from patricknavarromahoney/master

add jruby 1.7.14 build
add jruby-1.7.15
Merge pull request #624 from chulkilee/jruby-1.7.15

add jruby-1.7.15
ruby-build 20140905
Don't log files as they're extracted from tarball

The `-v` option for tar has been here since the beginning of ruby-build,
but it's not really informative to see a list of files in the log as the
list of files is guaranteed to be the same across machines since we do
checksums.
Print configure options when openssl ext failed to compile

This helps to debug whether `--with-openssl-dir` was used properly.
Indicate ruby-build version number on build failures

This might save us from having to ask people which version they have
installed when they paste us build output from the console.
Prevent "BUILD FAILED" appearing twice on `./configure` error

Since `./configure` executes in a bash subshell, it failing would
execute the ERR trap twice: once in a subshell and once in the main
process. An explicit `return 1` skips one of these ERR traps and fixes
double output.

I hope.

[image of I Have No Idea What I'm Doing dog]
Highlight output that mentions path to full build log

People often miss that and think that the last 10 lines of build output
is the only information they got.
Print OS information on build failure

This is useful info for when someone is pasting build output to our
issue tracker seeking support.
Fix output assertion

`assert_success` doesn't accept an argument via stdin
Fix `rbenv install --list` for definitions auto-discovery

The build definitions auto-discovered from rbenv plugins would
previously not appear in `--list` results due to discovery process
taking place too late.
Merge pull request #627 from sstephenson/list-custom-definitions

Fix `rbenv install --list` for definitions auto-discovery
Merge pull request #626 from sstephenson/build-output

Make build output and log more useful
Fix detecting of CPU cores on FreeBSD

On FreeBSD, use the same approach as OS X with `sysctl`.
Verify checksums for pull requests from other people

This has the CI step verify the checksums for URLs that might have been
added or edited to build definitions in a pull request.