chore: remove support for ember 3.4

BREAKING CHANGE: This removes support for ember 3.4 which is not a
supported LTS version anymore. This also adds a test matrix for the
latest LTS version 3.12

.travis.yml

@@ -46,8 +46,8 @@ 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-3.4
-    - env: EMBER_TRY_SCENARIO=ember-lts-3.8
+      env: EMBER_TRY_SCENARIO=ember-lts-3.8
+    - env: EMBER_TRY_SCENARIO=ember-lts-3.12
     - env: EMBER_TRY_SCENARIO=ember-release
     - env: EMBER_TRY_SCENARIO=ember-beta
     - env: EMBER_TRY_SCENARIO=ember-canary

config/ember-try.js

@@ -6,18 +6,18 @@ module.exports = async function() {
   return {
     scenarios: [
       {
-        name: "ember-lts-3.4",
+        name: "ember-lts-3.8",
         npm: {
           devDependencies: {
-            "ember-source": "~3.4.0"
+            "ember-source": "~3.8.0"
           }
         }
       },
       {
-        name: "ember-lts-3.8",
+        name: "ember-lts-3.12",
         npm: {
           devDependencies: {
-            "ember-source": "~3.8.0"
+            "ember-source": "~3.12.0"
           }
         }
       },