Update ruby-pkg-config to 1.2.3.

== 1.2.3 - 2017-05-29

=== Fixes

  * Supported *-mingw32 Rubies without MSYS2 again.

== 1.2.2 - 2017-05-29

=== Improvements

  * Supported path normalization for MSYS2.

== 1.2.1 - 2017-05-29

=== Improvements

  * Supported MSYS2.

== 1.2.0 - 2017-04-25

=== Improvements

  * Supported x32 environment. [GitHub#12][Reported by Elan Ruusam辰e]

=== Thanks

  * Elan Ruusam辰e

== 1.1.9 - 2017-04-20

  * Supported C++ compiler.

== 1.1.8 - 2017-04-19

  * Stopped to require mkmf.

(taca)