chore(ci): run tests against Ember 2.12+

Looking at other `ember-cli` packages, this seems to be what they
support.

As Ember moves forward and toward proper ES classes, supporting Ember
2.4 introduces compatibility issues.

BREAKING CHANGE: No longer committing to support for Ember versions less
than 2.12

.travis.yml

@@ -23,8 +23,6 @@ env:
   matrix:
     # we recommend new addons test the current and previous LTS
     # as well as latest stable release (bonus points to beta/canary)
-    - EMBER_TRY_SCENARIO=ember-lts-2.4
-    - EMBER_TRY_SCENARIO=ember-lts-2.8
     - EMBER_TRY_SCENARIO=ember-lts-2.12
     - EMBER_TRY_SCENARIO=ember-lts-2.16
     - EMBER_TRY_SCENARIO=ember-lts-2.18

config/ember-try.js

@@ -11,38 +11,6 @@ module.exports = function() {
     return {
       useYarn: true,
       scenarios: [
-        {
-          name: 'ember-lts-2.4',
-          bower: {
-            dependencies: {
-              ember: 'components/ember#lts-2-4'
-            },
-            resolutions: {
-              ember: 'lts-2-4'
-            }
-          },
-          npm: {
-            devDependencies: {
-              'ember-source': null
-            }
-          }
-        },
-        {
-          name: 'ember-lts-2.8',
-          bower: {
-            dependencies: {
-              ember: 'components/ember#lts-2-8'
-            },
-            resolutions: {
-              ember: 'lts-2-8'
-            }
-          },
-          npm: {
-            devDependencies: {
-              'ember-source': null
-            }
-          }
-        },
         {
           name: 'ember-lts-2.12',
           npm: {