Allow for bundler >1.13

[fenwanderer]

As @sethfarns reported in nbgallery/jupyter-alpine#12, the Jupyter "interrupt kernel" button began to fail after installing anything with iruby-dependencies. @kylekyle noted that bundler 1.14.0 broke iruby-dependencies, speculating that bundler as "exec'ing something in a way that confuses zmq" (see ipython/ipython#3400).

Also, "in bundler 1.13.7 interrupting a sleep works, but doesn't produce an Interrupt error message."

d125ebe temporarily fixed this by enforcing bundler >~1.13.0, but iruby-dependencies should probably still be updated to work with newer versions of bundler.