Requires Ruby >= 2.4.0 due to Filigree

With the new dependency on filigree, gem will now choke trying to install on Ruby versions lower than 2.4.0:

steven@macbook:~/workspace/apps/ios $ gem install rltk
Fetching: ffi-1.9.25.gem (100%)
Building native extensions.  This could take a while...
Successfully installed ffi-1.9.25
Fetching: filigree-0.4.1.gem (100%)
ERROR:  Error installing rltk:
	filigree requires Ruby version >= 2.4.0.```

It looks like it can be worked around with:

steven@macbook:~/workspace/apps/ios(sh/parallelize-test) $ gem install filigree -v 0.2
Fetching: filigree-0.2.0.gem (100%)
Successfully installed filigree-0.2.0
1 gem installed
Last command: 0.622s   Exit Code: 0

steven@macbook:~/workspace/apps/ios(sh/parallelize-test) $ gem install rltk
Fetching: rltk-3.0.1.gem (100%)
Successfully installed rltk-3.0.1
1 gem installed```