Require pry-nav in spec_helper

Already done in test_helper, enables pry when running a single
spec.

spec/spec_helper.rb

@@ -1,3 +1,9 @@
+begin
+  require 'pry-nav'
+rescue LoadError
+  # It's OK, we don't include pry in e.g. gemfiles/3.0.gemfile
+end
+
 # This file was generated by the `rspec --init` command. Conventionally, all
 # specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.
 # The generated `.rspec` file contains `--require spec_helper` which will cause this