`rbenv install` completion includes definitions from plugins

Before, `ruby-build --definitions` was invoked sooner than
RUBY_BUILD_DEFINITIONS was built up with paths from rbenv plugins.

[ci skip]

On FreeBSD 10, stop defaulting to MAKE=gmake

Sort Ruby versions when listing available definitions

Stable releases should now be sorted as a higher version than preview
releases or RCs. For instance:

- 1.9.3-preview < 1.9.3-rc1 < 1.9.3-p0
- 2.1.0-dev < 2.1.0-rc1 < 2.1.0
- jruby-1.7.0-preview1 < jruby-1.7.0-rc1 < jruby-1.7.0

Fixes #604

This detects when `/tmp` has been mounted with "noexec" mode and avoids
the cryptic error:

  ./configure: Permission denied

Hopefully the information provided will be enough for the user to know
that they have to change TMPDIR to another location.

It appears that regular `make` that ships on FreeBSD 10 is compatible
enough to build Rubies. This enables ruby-build on fresh FreeBSD
installs (which don't have `gmake` by default) without having to
explicitly set `MAKE=make`.

Closes #610

Now supports `-h|--help`.

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

On FreeBSD, use the same approach as OS X with `sysctl`.

Make build output and log more useful

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.

`assert_success` doesn't accept an argument via stdin

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

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

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]

This might save us from having to ask people which version they have
installed when they paste us build output from the console.

This helps to debug whether `--with-openssl-dir` was used properly.

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.

add jruby-1.7.15

add jruby 1.7.14 build

Read ruby-build revision from git if available

Avoid cloning the whole project's history which we'll never need

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.

Auto-discovery of build definitions from rbenv plugins

add ruby-1.9.2-p330

The `share/ruby-build/` directory from each rbenv plugin, if it exists,
is added to RUBY_BUILD_DEFINITIONS automatically during `rbenv install`.

Support extra definitions paths via RUBY_BUILD_DEFINITIONS

Closes #609