Update ruby-RMagick to 2.15.2.

## RMagick 2.15.2

- GitHub repository moved back to github.com/rmagick/rmagick - @wurde, @vassilevsky

## RMagick 2.15.1

- Fix loop in linked list in `ImageList` methods => they no longer hang - @u338steven

RMagick 2.15.0
    o Ability to remove alpha channel - @ollie
    o C local variables guarded against GC to avoid segfaults - @u338steven
    o trace_proc protected with a mutex to avoid segfaults - @u338steven

RMagick 2.14.0
    o RMagick.rb moved to deprecated directory - @mockdeep
    o Better ImageMagick feature detection - @bf4
    o Prevent compilation failures if prefix is an empty string - @voxik
    o SplineInterpolatePixel preprocessor check removed - @joshk
    o Make error format a string literal - @mtasaka
    o Automatically set ARCHFLAGS on OS X - @u338steven
    o Fixed 'rmagick/version' require failure - @u338steven
    o Fixed escaping of '%' in Draw#get_type_metrics - @mkluczny
    o Silence warnings on Ruby 2.2 when comparing Enums - @meanphil
    o Multiple test suite improvements - @mockdeep, @bf4
    o Ruby source code formatting with RuboCop - @vassilevsky

(taca)