revert: the previous terser fix and update to a new version (#14)

Fixes #12

index.js

@@ -9,7 +9,6 @@ const resolve = require('rollup-plugin-node-resolve');
 const {terser} = require('rollup-plugin-terser');
 const istanbul = require('rollup-plugin-istanbul');
 const path = require('path');
-const isCi = require('is-ci');
 
 const transformObjectAssign = require('@babel/plugin-transform-object-assign');
 const presetEnv = require('@babel/preset-env');
@@ -184,20 +183,14 @@ const getSettings = function(options) {
     settings.excludeCoverage = options.excludeCoverage(settings.excludeCoverage);
   }
 
-  const terserOptions = {output: {comments: 'some'}};
-
-  if (isCi) {
-    terserOptions.numWorkers = 1;
-  }
-
   // primed plugins
   settings.primedPlugins = {
     babel: babel(settings.babel),
     commonjs: commonjs({sourceMap: false}),
     json: json(),
     multiEntry: multiEntry({exports: false}),
     resolve: resolve({browser: true, main: true, jsnext: true}),
-    uglify: terser(terserOptions),
+    uglify: terser({output: {comments: 'some'}}),
     istanbul: istanbul({exclude: settings.excludeCoverage})
   };