Remove __dir__ usage from extension installation (#531)
Remove __dir__ usage from extension installation (#531)

The `__dir__` method was introduced in Ruby 2.0.
To support 1.9 we can't keep using it.

Instead use the `EXT_PATH` as a basis for the gem install path, which is
based on the `__FILE__` keyword that is available in Ruby 1.9.
appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby