Add missing imagemagick dependency to Docker image

Dockerfile

@@ -28,6 +28,7 @@ RUN apt-get update && \
       autoconf \
       build-essential \
       git-core \
+      imagemagick \
       libfontconfig1 \
       libpq-dev \
       libtool \
@@ -46,6 +47,7 @@ RUN apt-get update && \
 #   autoconf, libtool and pkg-config for libtsm
 #   libfontconfig1 for PhantomJS
 #   ttf-bitstream-vera for a2png
+#   imagemagick (identify) for PNG generator (Ruby)
 
 # install Bundler