Use debugger aliases only when pry-debugger is available.

Currently, if pry-debugger is not installed, adding these bits to
~/.pryrc makes it show a warning:
  error loading ~/.pryrc: undefined method `options' for
  nil:NilClass

This small tweak makes pry try to set up the debugger aliases only if
PryDebugger is available, thus hiding the error when it's not.
Merge pull request #37 from arnab/readme-tweaks

Use debugger aliases only when pry-debugger is available.