Fixes #440

Use the `cp -f` flag to force unlinking non-writable files.

Closes #384

When installing mruby-dev, the cp command doesn't use the force flag.
This causes an issue when using a custom build configuration (via
MRUBY_CONFIG environment variable) and installing mrbgems from Github,
as the pack files in the git repositories are read-only.

Helps avoid the pitfall where ERR trap has no effect.
See 262eb165

Previously, if `http get` failed, the `download_tarball` function would
still continue since the ERR trap had no effect at that point.

Given a script in the form of `{ ... } || return 1`, the expressions
that are the part of the first group are not subject to ERR trap since
they are non-last in a chain of expressions.

However, since we still can't count on the ERR trap taking effect in
this phase, better just rewrite the function to manually abort.

Fixes #394

Closes #293

Fixes #110

- use cache path feature to skip HTTP fetch atogether
- unified build.log

* Shorter.
* Preserve timestamps and permissions.
* Overwrite existing files.

`cd` into project's root on every invocation.

Fixes #243

Works on OS X but fails on CI.

Use Homebrew libyaml if available.

Closes #381

When stub patterns were broken down into an array, they were
accidentally subject to glob expansion. E.g., a pattern '*' might expand
to whatever is in the current directory. This is unwanted, as we need to
preserve patterns as-is.

No more awkward juggling of PATH entries and preset stubs in version
control. Commands are stubbed by symlinking the main stub from a
temporary `$TMP/bin` directory that's added once to PATH in tests.

Conflicts:
	share/ruby-build/1.9.1-p378
	share/ruby-build/1.9.1-p430
	share/ruby-build/1.9.2-p0
	share/ruby-build/1.9.2-p180
	share/ruby-build/1.9.2-p290
	share/ruby-build/1.9.2-p318
	share/ruby-build/1.9.2-p320
	share/ruby-build/1.9.3-p0
	share/ruby-build/1.9.3-p125
	share/ruby-build/1.9.3-p194
	share/ruby-build/1.9.3-p286
	share/ruby-build/1.9.3-p327
	share/ruby-build/1.9.3-p362
	share/ruby-build/1.9.3-p374
	share/ruby-build/1.9.3-p385
	share/ruby-build/1.9.3-p392
	share/ruby-build/1.9.3-p429
	share/ruby-build/1.9.3-p448
	share/ruby-build/1.9.3-preview1
	share/ruby-build/1.9.3-rc1
	share/ruby-build/2.0.0-preview1

Keep mirror up-to-date using Travis CI

Grep mustn't get invoked with 0 parameters, otherwise it will hang
waiting on stdin.

When new Ruby definitions get added, our current CloudFront mirror may
get out of date. This adds a task to the CI process that detects
added/changed package URLs and uploads them to our S3 bucket.

Fixes #392, references #395

Fixes breakage caused by curl arguments change in
d0912e4f

Fix Rubinius gem binstubs path

`tar` on OS X auto-detects the type of compression used, so it doesn't
care whether `-z` or `-j` flags were passed to it for extraction.
However, since latest Rubinius archives are compressed with bzip2, we
must handle this distinction explicitly for the sake of other platforms.

fixes #470

Added a definition for JRuby 1.7.6.

Add rbx-2.1.1

Rubinius 2 insists that it installs RubyGems binstubs into
`PREFIX/gems/bin` instead of `PREFIX/bin`.

This creates complexity for rbenv rehash and exec processes, so we
symlink `gems/bin` into `bin` and have RubyGems create binstubs at a
location that is consistent with other Ruby implementations.

See sstephenson/rbenv#178, sstephenson/rbenv#461

Add Rubinius 2.1.0