fix(MongoBinaryDownloadUrl): add Ubuntu 18.04 builds

* Ubuntu versions list updated

* Check major ubuntu version only

* Added unit test for Ubuntu 18.04 release

Author: YuriGor

.gitignore

@@ -44,3 +44,4 @@ node_modules
 
 coverage
 .nyc_output
+package-lock.json

src/util/MongoBinaryDownloadUrl.js

@@ -200,6 +200,8 @@ export default class MongoBinaryDownloadUrl {
       name += '1604';
     } else if (majorVer === 16) {
       name += '1604';
+    } else if (majorVer === 18) {
+      name += '1804';
     } else {
       name += '1404';
     }

src/util/__tests__/MongoBinaryDownloadUrl-test.js

@@ -117,6 +117,14 @@ describe('MongoBinaryDownloadUrl', () => {
         })
       ).toBe('ubuntu1204');
     });
+    it('should return a archive name for Ubuntu 18.04', () => {
+      expect(
+        downloadUrl.getUbuntuVersionString({
+          dist: 'Ubuntu Linux',
+          release: '18.04',
+        })
+      ).toBe('ubuntu1804');
+    });
   });
 
   describe('getDebianVersionString()', () => {