Remove TEST_ENV variable

Karma doesn't support loading ES modules, so all this was doing was
using Babel to translate everything back to CommonJS before testing
anyway, which kinda defeats the purpose of testing multiple builds.

.travis.yml

@@ -4,10 +4,6 @@ cache: yarn
 jobs:
   include:
   - stage: Test
-    env: TEST_ENV=source
-    script: yarn test
-  - env: TEST_ENV=umd
-    before_script: yarn build
     script: yarn test
   - stage: Size
     before_script: yarn build

karma.conf.js

@@ -62,16 +62,6 @@ module.exports = function(config) {
     // },
   };
 
-  var historyAlias;
-  switch (process.env.TEST_ENV) {
-    case 'umd':
-      historyAlias = 'umd/history.js';
-      break;
-    case 'source':
-    default:
-      historyAlias = 'modules/index.js';
-  }
-
   config.set({
     customLaunchers: customLaunchers,
     browsers: ['Chrome' /*, 'Firefox'*/],
@@ -94,7 +84,7 @@ module.exports = function(config) {
       },
       resolve: {
         alias: {
-          history$: path.resolve(__dirname, historyAlias)
+          history$: path.resolve(__dirname, 'modules/index.js')
         }
       },
       plugins: [