Update pgmagick requirement for Mac OS

Installation failed on Mac OS with pgmagick 0.5.5 (libboost library
could not be found because of an error in the install script). This was
fixed in 0.5.7
(https://bitbucket.org/hhatto/pgmagick/commits/977a2d1862553198d347464bb
7ca2ea7efcf6090)
Add ProjectStackInline to ProjectAdmin

ProjectAdmin now shows associated Stacks. Also renamed ProjectInline to
ProjectStackInline to better reflect the model name.
Merge remote branch 'felix/small-fixes'