parse the bundler version out of the lock

the old code was just using the version of Bundler that was running,
not actually reading the version of Bundler that was in the lockfile.
switch to a warning for old bundlers

we actually want to warn people that they should upgrade, not force them
to always have to upgrade if they want to install.
rename LockfileParser#bundler to bundler_version
rename Bundler.locked_gems to Bundler.lock

it's not actually a list of locked gems, it's the LockfileParser that
has parsed the current lock. so let's just call it a lock.
just use Bundler.lock.bundler_version

we already have a LockfileParser in Bundler.lock, so we can just use
that one to get the locking Bundler version, instead of writing a new
method that just gets us the Bundler version.
save the locking bundler version before re-locking

running the installer will write out a new lock file, because that
is what installing does. so we save the bundler version before the
installer runs, so we can check it later to print a warning about
being out of date.