[UPDATE] Drop support for Ruby 1.9.2

Arguments:

* Ruby 1.9.2 reached EOL (does not receive security fixes)
* Upgrading from 1.9.2 to 1.9.3 is trivial
* Most people on 1.9 are already on 1.9.3
* Maintaining support of Adhearsion and related projects on unmaintained Ruby versions is a disservice to our users. There are enough reasons for an upgrade regardless of Adhearsion's support (security issues, etc), suggesting that it's ok to continue using unmaintained language versions is poor advice, and attempting to maintain support distracts us from improving the project (security, bugs, performance and features) on modern maintained platforms.

I suggest that Adhearsion's versioning policy should state that external dependencies (Ruby versions, Asterisk/FS versions) will be supported within a major release of Adhearsion until the point at which they reach their own EOL date, at which point Adhearsion's support for those dependencies will cease to be guaranteed.

Review please @bklang.