Add ember-lts-2.12 back into ember-try scenarios

[juwara0]

Problem Description

Currently, the ember-lts-2.12 scenario in .travis.yml is failing in CI due to: ember-cli/ember-try#148

Error in Travis build: https://travis-ci.org/ciena-frost/ember-frost-notifier/jobs/289662299#L615

Missing npm packages: 
Package: ember-source
  * Specified: ~2.12.0
  * Installed: 2.16.0
Run `npm install` to install missing dependencies.
An error occurred in the constructor for ember-cli-dependency-checker at /home/travis/build/ciena-frost/ember-frost-notifier/node_modules/ember-cli-dependency-checker

Solution

Once ember-cli/ember-try#148 is resolved add:

- EMBER_TRY_SCENARIO=ember-lts-2.12

back to .travis.yml: https://github.com/ciena-frost/ember-frost-notifier/blob/master/.travis.yml#L23