Rename `to_json` to `to_json_string`

The method named `to_json` is pseudo-magic in Ruby; the json gem
defines it on every object, and relies on a zero argument `to_json`
method existing on everything nontrivial.  This is too much of a
headache to work around in the Ruby driver alone, so we are forced to
spread the misery and rename the method.

Which is tragic, because it breaks consistency, but that’s life.
Add aliases for Python and JavaScript
Merge branch 'gchpaco_3123' into next

Fixes #3123.
Use latest Ubuntu AMI as base in build AMI script
allow building with protobuf 2.6.0

OTS review by mlucy
Closes #3137
Merge branch 'next' into sam_2134_iii