Drop Ruby 1.8 support
windows: update bundled libraries versions
Add 1.12.7 release entry
windows: set PATH to use vendor/local/bin/libpng-config
windows: make fontconfig cross-compilable

Test is not considered about Windows. So I disable it.
windows: bundle GCC related DLLs
windows: support rake-compiler 0.9.1
Bump version

1.12.7 ->
1.12.8
     ^
widnows: bundle missing libwinpthread-1.dll
Add 1.12.8 entry
Bump version

1.12.8 ->
1.12.9
     ^
windows: add missing directory task