Drop support for ember < 3.8

Ember LTS 3.8 has bugfixes support until Sep 2019. So at the time of writing it's already a past date. But it's relatively easy to support that version.
When it comes to its ancestor LTS 3.4 it's not that straightforward.

Author: tniezurawski

.travis.yml

@@ -43,10 +43,6 @@ jobs:
     # we recommend new addons test the current and previous LTS
     # as well as latest stable release (bonus points to beta/canary)
     - stage: "Additional Tests"
-      - env: EMBER_TRY_SCENARIO=ember-lts-2.12
-      - env: EMBER_TRY_SCENARIO=ember-lts-2.16
-      - env: EMBER_TRY_SCENARIO=ember-lts-2.18
-      - env: EMBER_TRY_SCENARIO=ember-lts-3.4
       - env: EMBER_TRY_SCENARIO=ember-lts-3.8
       - env: EMBER_TRY_SCENARIO=ember-release
       - env: EMBER_TRY_SCENARIO=ember-beta

README.md

@@ -101,8 +101,8 @@ var buffer = ObjectProxy.extend(BufferedMixin).create({
 Compatibility
 ------------------------------------------------------------------------------
 
-* Ember.js v2.12 or above
-* Ember CLI v2.12 or above
+* Ember.js v3.8 or above
+* Ember CLI v3.8 or above
 * Node.js v8 or above
 
 Contributing

config/ember-try.js

@@ -6,50 +6,6 @@ module.exports = async function() {
   return {
     useYarn: true,
     scenarios: [
-      {
-        name: 'ember-lts-2.12',
-        env: {
-          EMBER_OPTIONAL_FEATURES: JSON.stringify({ 'jquery-integration': true }),
-        },
-        npm: {
-          devDependencies: {
-            '@ember/jquery': '^0.5.1',
-            'ember-source': '~2.12.0'
-          }
-        }
-      },
-      {
-        name: 'ember-lts-2.16',
-        env: {
-          EMBER_OPTIONAL_FEATURES: JSON.stringify({ 'jquery-integration': true }),
-        },
-        npm: {
-          devDependencies: {
-            '@ember/jquery': '^0.5.1',
-            'ember-source': '~2.16.0'
-          }
-        }
-      },
-      {
-        name: 'ember-lts-2.18',
-        env: {
-          EMBER_OPTIONAL_FEATURES: JSON.stringify({'jquery-integration': true}),
-        },
-        npm: {
-          devDependencies: {
-            '@ember/jquery': '^0.5.1',
-            'ember-source': '~2.18.0'
-          }
-        }
-      },
-      {
-        name: 'ember-lts-3.4',
-        npm: {
-          devDependencies: {
-            'ember-source': '~3.4.0'
-          }
-        }
-      },
       {
         name: 'ember-lts-3.8',
         npm: {