Start using VCR to interact with external APIs servers to keep cached copies of the responses and save time.

Note: For now VCR will be used to save time, but in the future the cassettes could be added to source control to ensure repeatability. However, cassettes should be re-recorded from time to time to make sure the external APIs haven't done any breaking change. Until decided how the cassettes will be maintained, them will be kept out of source control and will last only 1 day on developers' computers.

Improvements stats:

BEFORE:
=======
$time rake
...
Finished in 4 minutes 3.8 seconds (files took 11.83 seconds to load)
2800 examples, 17 failures, 169 pending

real	4m28.518s
user	3m18.656s
sys	0m10.216s

AFTER:
======
$time rake
...
Finished in 3 minutes 39.7 seconds (files took 11.26 seconds to load)
2800 examples, 17 failures, 169 pending

real	4m4.360s
user	3m18.756s
sys	0m9.404s

---

BEFORE:
=======
$ time rspec ./spec/models/georeference/geo_locate_spec.rb
...
Finished in 35.05 seconds (files took 8.01 seconds to load)
19 examples, 0 failures
[Coveralls] Outside the Travis environment, not sending data.

real	0m46.572s
user	0m18.532s
sys	0m1.692s

AFTER:
======
$ time rspec ./spec/models/georeference/geo_locate_spec.rb
...
Finished in 11.39 seconds (files took 7.89 seconds to load)
19 examples, 0 failures
[Coveralls] Outside the Travis environment, not sending data.

real	0m22.600s
user	0m18.424s
sys	0m1.808s