Andrei - I don't know if this helps, but your comments were so helpful I wanted to mention this. I've had ruby installed and didn't see a lot of additional dependencies. I just now installed ruby-irb and saw that opkg installed a whole bunch of ruby-related stuff.

opkg install ruby-irb
Installing ruby-irb (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-irb_2.2.4-1_ar71xx.ipk.
Installing ruby-debuglib (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-debuglib_2.2.4-1_ar71xx.ipk.
Installing ruby-multithread (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-multithread_2.2.4-1_ar71xx.ipk.
Installing ruby-misc (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-misc_2.2.4-1_ar71xx.ipk.
Installing ruby-prettyprint (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-prettyprint_2.2.4-1_ar71xx.ipk.
Installing ruby-filelib (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-filelib_2.2.4-1_ar71xx.ipk.
Installing ruby-enc (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-enc_2.2.4-1_ar71xx.ipk.
Installing ruby-math (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-math_2.2.4-1_ar71xx.ipk.
Installing ruby-patterns (2.2.4-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/packages/ruby-patterns_2.2.4-1_ar71xx.ipk.
Configuring ruby-enc.
Configuring ruby-misc.
Configuring ruby-filelib.
Configuring ruby-multithread.
Configuring ruby-patterns.
Configuring ruby-math.
Configuring ruby-prettyprint.
Configuring ruby-debuglib.
Configuring ruby-irb.