Update README.txt

* add ruby20 and ruby21 rake tasks into quick start
Merge pull request #220 from simi/patch-1

Update README.txt
Upgrade Ruby 2.0.0 to patchlevel 481

Results of 'make check':
https://gist.github.com/luislavena/628b43c0494b560aa256
Match Quick start instructions
Add mingw64 4.8.2-rev0 x86 and x64 support
Only add DLDFLAGS for relevant DevKit's

Current GCC 4.8.x compilers add an implicit dependency to the libgcc
support library to msvcrt-rubyXYZ.dll. This overly complicates
installs and creates additional support concerns. Adding -static-libgcc
to configure's DLDFLAGS "fixes" this GCC behavior. Rather than
hard-coding DLDFLAGS for every build, set only when appropriate.

NOTE: while generic to 4.8.x, the DevKit version matching regex must
be updated to support future GCC versions if this GCC behavior persists.
Upgrade mingw64-{32,64}-4.8.2 to rev 1
Update mingw64-{32,64}-4.8.2 to rev2
Update mingw64-{32,64}-4.8.2 to rev3
Update mingw64-{32,64}-4.8.2 to rev4
Add mingw64-{32,64}-4.9.0 rev1 support
Use msys2(32bit) instead of old msys

Parallel make hangs due to msys bug.
http://sourceforge.net/p/mingw/bugs/1950/

msys2 is updated version of msys and the bug seems to be fixed.
http://mingw-users.1079350.n2.nabble.com/Status-of-MSYS2-td7582391.html

We pick up msys2 minimal dependencies to build ruby.
Need to set MSYSTEM=MINGW32 environment variable.

Default value of MSYSTEM is changed with msys2.
http://sourceforge.net/p/msys2/tickets/4/
Pass make options by MAKE_OPT environment variable

We can pass -jN to make for build speedup.
Replace mingw/gnuwin32 packages to msys2

Add perl for autoconf
Add missing host option of x86_64 build
Keep mintty in devkit